T h e next stage is shown in Fig. Posterior part of oesophagus reduces in diameter and has deeply furrowed epithelium. I have added two figures for the sake of orientation. T h e observations will be checked by information gathered from serial sections. T h e region between the constriction and the circular band is of about t h e same length as before, but shows faint surface lines that correspond to the posterior border of the collar. This was partly owing to the difficulty I had a t first in getting sufficient numbers of the adult worms. On the basis of other developmental differences the relationship of these two groups has been rejected.
The funnel-like anterior part of the collar, the collarette, embraces the proboscis stalk and usually also the posterior part of the proboscis. The nervous system is thus essentially epidermal in position and diffused, an interesting concentration of nerve-cells and fibres. Pygochord is longitudinal rod-like structure extending from the ventral side of the intestine to the body wall, its function is not known but it may support the soft abdominal region. The animal moves inside the burrow. A much later stage in t h e history of the proboscis vcsicle is shown in Figs. The collar coelom and the trunk coelom are separated by collar- trunk septum.
T h e anterior pair run almost to the knobs of the eyes. Small portions of the circum-oral or extra-oral area a r e shown in Figs. Of these the first two communicate with the exterior by means of a pair of ciliated pore-canals placed at the posterior end of their respective segments. There is no central nervous system in Balanoglossus. T h e village Alicetown is situated o n the south-west corner of the horse-shoe formed by North Bimini, and therefore on the Stream side.
Therefore, some recent workers like Vander Host 1939 Dowydoff 1948 , Marcus 1958 and Hymen 1959 have chosen to remove hemichordates from the phylum chordata and treat them as an independent invertebrate phylum. Around the cavity is a protoplasmic layer containing nuclei, and t h e outer portion of the notochord is formed of t h e clear vacuolated ends of thcsc cells. The body and are divided into three sections, the middle section bearing gill slits. T h e posterior ciliated band has grown broader, and is marked by dark, black pigment specks. The largest acorn worm Balanoglossus gigas reaches is more than 2.
A t a later stage in development they took into their digestive tracts large fragments of the coral sand, completely filling and distending the walls of the tube. On this account the animal was subsequently placed by Gegenbaur in a special class of Vermes, the Enteropneusta. What the function of this knob may be I do not know. We also see that the tongue bar is formed from a portion of the upper lateral wall of the endodermal gillpouch. Though most authors admit that there are more resemblances to chordates, but larval similarities auricularia larva of echinoderms and tornaria larva of enteropneusts and molecular analysis draw a closer link between echinoderms and hemichordates rather than hemichordates and other chordates. T h e muscles serve probably as t h e retractors of the proboscis. T h e termination of the band is in stained specimens definitely made o u t ; in fact t h e very last nucleus of the band is easily separated from the surrounding nuclei of t h e apical plate.
One major morphological difference between chordates and annelids or arthropods is the opposite orientation of the nerve cord and heart. . T h e digestive tract is pulled still farther posteriorly so that the gill-pouches now lie a t a level posterior t o the constriction. The first cleavage starts about two hours after fertilisation and produces two generally, but not invariably, equal cells. If we examine in detail the apex of the larva we find the ectoderm thickened, forming an apical plate containing two No. The proboscis coelom opens out through the proboscis pore situated mid-dorsally near its base. The collar tissue contains two distinct lacunar networks which communicate posteriorly with a ring vessel.
Some of the cells of the eye are, therefore, inverted. The collar is also muscular and possesses two coelomic cavities. The proboscis-pores are highly variable, and frequently only one is present, that on the left side; sometimes the pore-canals of the proboscis unite to open by a common median orifice, and sometimes their communication with the proboscis coelom appears to be occluded, and finally the pore-canals may be quite vestigial. Its movement is made by a peculiar folded ridge of skin which carries long cilia. T h e number of gill-slits opening to the exterior i: :even.
A t this stage they reach as far forward as t h e middle of the collar region. You are going to email the following Asexual reproduction in Balanoglossus Stomochordata. The remaining posterior part of the archenteron marks the future gut or alimentary canal. As compared with the preceding stage, it will be seen that this region of the body has not only relatively but absolutely increased in length antero-posteriorly. This article has been rated as Stub-Class on the project's. Their walls are composed of ciliated epithelium lined externally by basement membrane and devoid of muscle fibres.
The two cavities are partitioned by incomplete mid-dorsal and mid-ventral mesenteries. The primary pharyngeal bars and the tongue bars are supported by skeletal rods. These regenerate completely into the adult sexual type found in winter. Digestive and Respiratory Systems 10. Both in tne tongue-bars and in the dividing-bars or betwccnbars the chitin-like skeleton of the gill-region is laid down by the endodermal cells, as shown in Fig. A central plate of ectoderm is in process of sinking beneath the surface.