188 



TUNICATA 



Hum (endoderm), and contains two or more cavities which 

 are continuous with blood sinuses in the mantle. In the 



Subneur- dorsal median line near the anterior end of the body, and 



al gland, imbedded in the mantle on the ventral surface of the nerve 

 ganglion, there lies a small glandular mass the subneural 

 gland which, as Julin has shown (?./), there is reason to 

 regard as the homologue of the hypo- 

 physis cerebri of the vertebrate brain. 

 Julin and E. van Beneden have sug- 

 gested that the function of this organ 

 may possibly be renal. 1 The sub- 

 neural gland, which was first noticed 

 by Hancock, communicates anteriorly, 

 as Ussoff (23) pointed out, by means 

 of a narrow duct with the front of 

 the branchial sac (pharynx). The 

 opening of the duct is enlarged to 

 form a funnel-shaped cavity, which 

 may be folded upon itself, convoluted, 

 or even broken up into a number of 



Dorsal smaller openings, so as to form a 



tubercle, complicated projection, called the 

 dorsal tubercle, situated in the dorsal 

 part of the prebranchial zone (fig. 7). 

 The dorsal tubercle in A. mentula is 

 somewhat horse-shoe-shaped (fig. 8) ; 

 it varies in form in most Ascidians 

 according to the genus and species, 

 and in some cases in the individual 

 also. Possibly, besides being the 

 opening of the duct from the sub- 

 neural gland, it may be a sense-organ 

 for testing the quality of the water 

 entering the branchial sac. 



Nervous The single elongated ganglion in 



system. ^ e median dorsal line of the mantle 

 between the branchial and atrial si- 

 phons is the only nerve-centre in A. mentula and most other 

 Tunicata. It is the degenerate remains of the anterior 



iq. 7. Diagrammatic sec- 

 tion through anterior dor- 

 sal part of A. mentula, 

 showing the relations of 

 the nerve ganglion, sub- 

 neural gland, &c. Letter- 

 ing as for fig. 4 ; , nerve ; 

 ', myelon ; pp, peripha- 

 ryngeal band ; sgl, sub- 

 neural gland ; sgil, its 

 duct ; t, test lining branch- 

 ial siphon. (Original.) 



FIG. 8. Dorsal tubercle and neighbouring organs of A. mentula.. Lettering 

 as before ; egr, epibranchial groove ; 2, prebranchial zone. (Original.) 



part of the cerebro-spinal nervous system of the tailed 

 larval Ascidian (see below, p. 614). The posterior or 

 spinal part has entirely disappeared in most Tunicata. 

 It persists, however, in the Appendiculariidx, and traces of 

 it are found in some Ascidians (e.g., Clavelina ; see Julin). 

 The ganglion gives off distributory nerves at both ends, 



1 See also Herdman, Future, vol. xxviii. p. 284. 



which run through the mantle to the neighbourhood of the Sense 

 apertures, where they divide and subdivide. The only or s an 

 sense-organs are the pigment spots between the branchial 

 and atrial lobes, the tentacles at the base of the branchial 

 siphon, and possibly the dorsal tubercle and the languets 

 or dorsal lamina. These are all in a lowly developed con- 

 dition. The larval Ascidians on the other hand have well- 

 developed intra-cerebral optic and auditory sense-organs ; 

 and in some of the pelagic Tunicata otocysts and pigment 

 spots are found in connexion with the ganglion. 



The mouth and the pharynx (branchial sac) have already Alimi 

 been described. The remainder of the alimentary canal ar y 

 is a bent tube which in A. mentula and most other Ascid- canal 

 ians lies imbedded in the mantle on the left side of the 

 body, and projects into the peribranchial cavity. The 

 oesophagus leaves the branchial sac in the dorsal middle 

 line near the posterior end of the dorsal lamina (see fig. 

 4, ceo). It is a short curved tube which leads ventrally 

 to the large fusiform thick-walled stomach. The intestine 

 emerges from the ventral end of the stomach, and soon 

 turns anteriorly, then dorsally, and then posteriorly so as 

 to form a curve the intestinal loop open posteriorly. 

 The intestine now curves anteriorly again, and from this 

 point runs nearly straight forward as the rectum, thus com- 

 pleting a second curve the rectal loop open anteriorly 

 (see fig. 4). The wall of the intestine is thickened inter- 

 nally, to form the typhlosole, a pad which runs along its 

 entire length. The anus opens into the dorsal part of the 

 peribranchial cavity near to the atrial aperture. The walls 

 of the stomach are glandular; and a system of delicate 

 tubules with dilated ends, which ramifies over the outer wall 

 of the intestine and communicates with the cavity of the 

 stomach by means of a duct, is probably a digestive gland. 



A mass of large clear vesicles which occupies the rectal Excr< 

 loop, and may extend over the adjacent walls of the in- tor y 

 testine, is a renal organ without a duct. Each vesicle is orgau 

 the modified remains of a part of the primitive crelom or 

 body-cavity, and is formed of cells which eliminate nitro- 

 genous waste matters from the blood circulating in the 

 neighbouring blood-lacuna? and deposit them in the cavity 

 of the vesicle, where they form a concentrically laminated 

 concretion of a yellowish or brown colour. These concre- 

 tions contain uric acid, and in a large Ascidian are very 

 numerous. The nitrogenous waste products are thus de- 

 posited and stored up in the renal vesicles in place of 

 being excreted from the body. In other Ascidians the 

 renal organ may differ from the above in its position and 

 structure ; but in no case has it an excretory duct, unless 

 the subneural gland is to be regarded as a renal organ. 



The heart is an elongated fusiform tube placed on the Bloot 

 ventral and posterior edge of the stomach, in a space (the vascu 

 pericardium) which is part of the original ccelom or body- s ^* te 

 cavity, the rest of which exists merely in the form of lacunae cfle i ol: 

 and of the cavities of the reproductive organs and renal 

 vesicles in the adult Ascidian. The wall of the heart is 

 formed of a layer of epithelio-muscular cells, the inner 

 ends of which are cross-striated ; and waves of contraction 

 pass along it from end to end, first for a certain number of 

 beats in one direction and then in the other, so as to reverse 

 the course of circulation periodically. At each end the 

 heart is continued into a vessel (see fig. 9), a large sinus 

 or lacuna lined with a delicate endothelial layer. The 

 sinus leaving the ventral end of the heart is called the 

 branchio-cardiac vessel, 2 and the heart itself is merely the 

 differentiated posterior part of this sinus and is therefore 

 a ventral vessel. The branchio-cardiac vessel, after giving 

 off a branch which, along with a corresponding branch from 

 the cardio- visceral vessel, goes to the test, runs along the 



2 On account of the periodic reversal of the circulation none of the 

 vessels can be called arteries or veins. 



