1220 



TUNICATA. 



The superior abdomen in Amarondum is 

 short and rounded, and slightly separated from 

 the thorax. The oesophagus is very short ; 

 the stomach in A. prolifcrum is marked ex- 

 ternally with a series of vertical folds, the 

 edges of which, seen under the microscope, 

 are furnished with secretory follicles. In A. 

 argus, the exterior of the stomach is coated 

 with a tessellated series of irregularly hexa- 

 gonal, gland-like, bodies, having the appear- 

 ance of a number of minute honeycomb com- 

 partments, around each of which are radiating 

 lamella? or folds. In Sidrtyum the stomach 

 is also surrounded with glands ; and in this 

 genus the intestine is spirally folded. In 

 JJiazona the stomach is striated externally ; 

 its inner surface is provided with numerous 

 salient folioles ; and the pyloric entrance is 

 guarded with an annular fold or valvule. The 

 intestine in its first part is simply membran- 

 ous, but afterwards it is furnished with irre- 

 gularly disposed glands, which have the form 

 of caeca! tubes. The stomach of Sigillma 

 bears some slight ridges on its inner surface, 

 the strongest of which correspond to external 

 furrows. In Aplidhtm the stomach is some- 

 what oblong and truncate, and is divided by 

 deep plications into three longitudinal cavities, 

 or rather into five, the lateral ones being sub- 

 divided. The intestine is bent sometimes 

 forward and sometimes backward ; in the 

 latter case it passes upward obliquely on the 

 right side of the stomach ; the rectum is 

 sometimes spirally arranged, and sometimes 

 straight. In Polyclinum also the intestine is 

 spirally folded, obliquely traversing the left 

 side towards the anal orifice. In A. proli- 

 fenim and A. argus the rectum ascends to 

 the middle of the cloaca. The post-abdo- 

 men is elongated, narrow, and tapering, 

 divided from the digestive cavity by a marked 

 constriction. The ovary occupies nearly the 

 whole length of this cavity (fg. 782. p), 

 and is partially surrounded by the glandular 

 mass of the testicle, from which latter an 

 undulatory vas deferens runs upwards to 

 end in the cloaca near the termination of 

 the rectum. The ova are whitish, yellow, 

 or brown, and on escaping from the ovary 

 appear to pass to the cloaca by the vacant 

 space between the intestines and the interior 

 of the mantle. The heart is a tubular vesicle, 

 bent on itself, and situated at the inferior 

 apex of the ovary. 



In the Didemnians the digestive, generative, 

 and circulatory organs are grouped together 

 in the pedunculated abdomen, and offer no- 

 thing remarkable. The ovary is situated by 

 the side of the intestine, and protrudes down- 

 wards only when full of eggs, which in this 

 case have, when fully developed, a large size, 

 compared with that of the animals. The 

 mantle of the Didemnum, as in many other 

 BotryllidcE, is frequently produced in the ab- 

 dominal region into stolon-like processes, 

 which traverse the common test. These swell 

 at their extremities into reproductive buds, 

 from whence arise new animals. Of these proli- 

 ferous tubes we shall have to speak hereafter. 



The stomach ofSotri/lloidesroHfcra(fg.783.) 

 is pyriform and divided into seven or eight lobes 

 by furrows that pass horizontally from the car- 

 diac to the pyloric extremity. The intestine 

 is sigmoid, and is divisible into three portions ; 

 the first part is smooth and transparent, the 

 second is surrounded with a granular tissue, 

 and the third is membranous like the first. 

 A glandular mass, apparently the hepatic 

 organ, is situated at the commencement of the 

 third portion of the intestine, and gives rise to 

 many minute excretory ducts that soon unite 

 into a single trunk, which appears to empty 

 itself into the intestine near the pylorus. Be- 

 low this organ and more behind, is another 

 glandular mass, which apparently belongs to the 

 generative organs, and gives origin to a little 

 duct passing upwards to the cloaca. The 

 heart is situated laterally, reposing on the 

 cardiac side of the stomach. In B. rubrum 

 the ovary is double, one part being situated 

 on either side of the thoracic chamber. The 

 testicle is seldom very distinct in the Didem- 

 nina and Botryllma, but has been occasionally 

 observed. 



> The nerve-ganglion between the two orifices, 

 or rather on the dorsal side of the buccal 

 orifice, is generally more or less distinct in 

 the Botryllidce. 



The circii/afion of the Botryllidce does not 

 materially differ from that of the other Asci- 

 dians, and has the same peculiar periodical 

 change in the direction of the blood-currents. 

 In the Polyclinina the heart is placed quite at 

 the inferior extremity of the post-abdomen. 

 It is invested with a thin, transparent pericar- 

 dium, and has the form of a large contractile 

 tube, bent on itself, and tapering at the ex- 

 tremities. In the Didemnina the heart is 

 shorter, and instead of being situated beneath 

 the ovary, it is lodged with that organ, by the 

 side of the intestinal loop, this condition 

 approaching that existing in the Clavellinides. 

 Lastly, in the Botryllina, it ascends still higher, 

 being seated near the stomach, nearly at the 

 base of the branchial sac. Milne-Edwards, 

 to whom we are indebted for the preceding 

 facts, remarks that these different positions of 

 the heart always coincide with analogous 

 changes in the position of the ovarium. It is 

 also the same, says he, in the simple Asci- 

 dians ; and Cuvier was, without sufficient 

 reasons, led to consider that the heart fol- 

 lowed in its position that of the mouth. 



If, says M. Edwards, we separate from the 

 common test of any of the Polyclinian species 

 a lively individual, and place it under the mi- 

 croscope with a little sea-water, the move- 

 ments of the heart may be easily studied. 

 The heart's contractions succeed each other 

 somewhat regularly, but they are not brisk 

 and extending at once through the organ, as 

 in the generality of animals. The contraction 

 commences at one of its extremities, and the 

 narrowing of the tube is propagated in an 

 undulatory manner towards the opposite ex- 

 tremity, in a manner somewhat similar to the 

 peristaltic movements of the intestines in the 

 higher animals. For some time the contrac- 



