TUiNICATA. 



1201 



and in the base of which opens the mouth; 

 that of the peritoneum, which does not com- 

 municate with the exterior by itself, but is 

 traversed by the intestinal tube, which, arising 

 in the branchial cavity, communicates with 

 the exterior by the rectum and the anal aper- 

 ture of the test ; and, lastly, that of the peri- 

 cardium, which has no direct communication 

 with the exterior. 



The position of the animal is always such, 

 that of the two orifices the branchial is always 

 the highest ; the entrance into the branchial 

 sac being generally placed at or near the 

 superior extremity of the body, and the oeso- 

 phageal opening, at the base of the branchial 

 sac, having an upward direction. In Boltenia 

 and Cystbigia the flexible peduncle, which is 

 attached at the summit of the body, above the 

 branchial orifice, allows the body to droop, 

 thus giving the animal its normal position. In 

 the Clavellinidcc, which have rigid peduncles 

 continuous with the base of the test, the same 

 normal relation of the internal parts is pre- 

 served ; and although the intestinal loop in 

 Boltenia and Clavellina is always directed to- 

 wards the pedicle, yet this arises from the loop 

 having, in the former an ascending, and in the 

 latter a descending, direction. 



The muscular sac or mantle, enclosed in 

 the external envelope, is attached to the in- 

 ternal surface of the latter by an adhesion 

 of the external surface of the extremities of 

 its two tubular processes, which correspond 

 to the two external orifices of the test, to 

 the inner borders of these outer orifices 

 (Jig. 780.). In Ascidia there is often no in- 

 timate cohesion at these points, merely an 

 adaptation easily disturbed after death. In 

 Cynthia, Boltenia, &c., on the contrary, the 

 test and the tubular prolongations are strongly 

 adherent. Beside this mode of attachment, 

 there is sometimes a general loose adhesion 

 formed by the epithelial tissue between the 

 surfaces of the test and mantle ; and in Cynthia 

 papillata there is a partial transmission of mus- 

 cular fibres from the latter to the former ; 

 and, lastly, there is the very slight attachment 

 arising from the passage of bloodvessels from 

 the body to the test. 



This free condition of the mantle within 

 the external sac occurs throughout the Asd- 

 diadce, Clavellinidce, Botryllidce, and Pyroso- 

 midce. In the Salpidce and the Pelonaiadce, 

 on the other hand, the mantle and the test 

 are connected at very many points, or even 

 throughout the extent of their contiguous 

 surfaces. 



The mantle in Chelyosoma is more closely 

 attached to the internal surface of the test 

 than in other Ascidians ; its muscular tissue 

 forming intimate connections between the 

 various plates of the external envelope, and 

 not only supplying the valvular pieces of the 

 apertures, but also edging each of the larger 

 plates with interlacing connecting fibres. Ex- 

 cept at these muscular spaces the mantle ap- 

 pears as a thin serous membrane. 



The interval between these two sacs is, 

 during life, filled with some fluid; possibly a 



VOL. IV. 



secretion, or transudation through the one 

 or the other of these envelopes; for, except 

 in the case of those Ascidians whose mantle- 

 tubes are not intimately connected with the 

 test, the sea-water cannot be directly admitted 

 into this cavity. 



The mantle is reflected upon the body, 

 properly so called, and covers it externally, 

 just as the peritoneum, after having coated 

 the walls of the abdomen, is reflected upon 

 the intestines ; with this difference, however, 

 that it has no mesentery, and that the con- 

 nection is only at the two orifices. It has 

 an external serous layer, continuous with the 

 internal membrane of the test, a muscular 

 tissue more or less extensive, and an internal 

 serous layer. It has also numerous ramifica- 

 tions of nerves and bloodvessels. In Boltenia, 

 Cystingia, and probably Bipapillaria, a tubu- 

 lar prolongation of the mantle traverses the 

 peduncle. 



The two tubes on the superior aspect of 

 the mantle, which are directed towards, and 

 protruding into, the two somewhat tubular 

 orifices of the test, are more muscular than 

 the rest of the sac, being surrounded with, 

 generally very distinct, sphincters in addition 

 to the fascicles of muscles that traverse them 

 longitudinally, and are continued diagonally 

 across the sac (Jig. 780.). When open, the 

 margins of these tubes are crenulate. The 

 free extremity of the branchial orifice is some- 

 times quite entire, but occasionally terminates 

 in a circle of regular tooth-like processes, 

 which are regarded bv Dr. A. Farre as the ana- 

 logues of the tentacles of the Bryozoa. By con- 

 traction, the tubes are thrown into 5 or 6 folds. 

 At the exterior angles of these folds in some 

 Ascidians ( A. intestinalis, Cynthia ampulla, &c.) 

 there is a minute red spot ; in A. mammilla/a, 

 the edges of the folds are more or less red- 

 dened by an increase and diffusion, as it were, 

 of these little red granules. Internally each 

 tube has frequently at its base some slight 

 valve-like prominences or folds ; and the in- 

 ner extremity of the branchial tube is alwajs 

 fringed with a circlet of simple or compound 

 tentacular filaments. These, however, may 

 more properly be said to belong to the bran- 

 chial sac. 



The mantle becomes dusky and opaque, 

 and its muscular tissue more distinctly 

 seen, in preserved specimens. In its recent 

 state it is more or less diaphanous, and 

 usually of a sober tint ; but occasionally, in 

 AscidicB, it is of a fine crimson (A. venosa), or 

 variegated with crimson and white (A. virgi- 

 nca}, or spotted with red (A. aspersa) ; and 

 in A. parallelogramma it is ornamented with 

 rectangular reticulating white lines and occa- 

 sional bright yellow or crimson spots. In A. 

 conchilega it is white, passing to blue ; and in 

 A. arachnoidca it is dark blue. 



The branchice of the Ascidiadce is a large 

 bag of fine vascular network furnished with 

 vibratile cilia, contained within the mantle of 

 the animal, and lining the walls of that 

 cavity (Jig. 778.). Sometimes the branchial 

 sac is oblong, oval, or rectangular ; and then 



4 ii 



