TUNICATA. 



1213 



the vitellus lost its mulberry aspect, and 

 when the albumen was formed. It closely 

 envelopes the caudal appendage, and termi- 

 nates beyond it by a tapering prolongation : 

 subsequently it becomes the test of the ani- 

 mal. The larva, resembling in appearance 

 a frog-tadpole, still remains a while in the 

 cloacal pouch, but is soon carried out by 

 the current leaving the animal : it is very 

 active, wriggling about for hours. After a 

 brisk locomotive existence for about twelve 

 hours, the embryo fixes itself to a foreign 

 body. The tail then begins to disappear, 

 being slowly absorbed; its pellucid membrane 

 remains entire fora little time, but is ultimately 

 detached from the body. At the same time 

 other appendages spring from the body, the 

 principal one of which arises from the an- 

 terior extremity, elongates itself more and 

 more, attains a trumpet-like shape, and may 

 be considered as the future mouth (fig. 

 781. G). In Amaroucium, Milne-Edwards ob- 

 served generally five appendages arising from 

 the anterior extremity of the body, two of 

 which soon disappear, and the three others, 

 which are persistent for a while, terminate 

 in button-shaped, dilated suckers, touching 

 the parietes of the external integument. In 

 Cynthia ampulla other processes or appendages 

 occur all over the body ; these are somewhat 

 similar to the anterior appendage, but are 

 never constant, either in direction or number; 

 one specimen had seven appendages on one 

 side. Sometimes the numerous appendages 

 resemble tentacles, giving the embryos the 

 appearance of young Tubiilarus. 



These processes are hollow, communicating 

 with the central cavity. None of them appear 

 to open externally, or to be at all like suckers; 

 and a considerable space frequently exists 

 between them and the enveloping membrane. 



The young animals are attached to solid 

 bodies by this external tunic, on a different 

 side to that at which the orifices make their 

 appearance. " If," says Van Beneden, " the 

 appendages served as suckers for attachment, 

 the animals would have the mouth on the side 

 by which they are attached." 



It is at this stage of growth that the walls of 

 the internal cavity are distinctly seen through 

 the external membranes. The contents of the 

 digestive tube are seen to consist of a rather 

 opaque mass, distinct from the surrounding 

 clear space. 



As soon as the animal is fixed, the trans- 

 parent membrane that has remained floating 

 since the absorption of the caudal appendage, 

 is completely detached, and the integument 

 and the test begin to be formed. 



In the third period of development the 

 embryo takes on the ascidian character. The 

 development of the internal organs progresses, 

 and the external orifices become apparent 

 (_y%. 781. H). At the commencement of this 

 period, the form of the young Ascidian varies 

 considerably. The numerous appendages dis- 

 appear, just as the caudal appendage was lost. 

 The embryo becomes rounded and larger, and 

 its substance is distinctly seen to be disposed 



in three layers. Up to the present time the 

 vitelline cavity has had no external communi- 

 cation, but the mucous cavity is now extended 

 on one side, to form the mouth. The mucous 

 layer is elongated also on the opposite side, 

 but instead of reaching the exterior, it is folded 

 back on itself, forming the intestinal loop. 

 The yolk soon opens a passage on each side, 

 constituting the oral or branchial, and the 

 ventral or anal, orifices. The intestinal tube 

 becomes completed by its separation into 

 the respiratory, the digestive, and the cloacal 

 cavities. Around each external orifice certain 

 nipple-like, palpiform bodies appear, which 

 soon lengthen, but before long they are 

 hidden by the integument (Jig. 781. i). The 

 eye-like pigmentary spot still continues for 

 some little time, and is situated in the middle 

 of a band (the cesophageal collar) that seems 

 to embrace the respiratory cavity. At the 

 base of the intestinal cavity, the opaque 

 concretionary body appears, to which the 

 heart is subsequently attached, and which Van 

 Beneden considers to be analogous to the 

 internal shell of the Limaces. This is furnished 

 with vibratile cilia, which occasion circulatory 

 currents, until the heart makes its appearance 

 in the form of a thin, gently pulsating, mem- 

 branous body. Contemporaneously with these 

 cilia, others appear in the interior of the bran- 

 chial sac, in the form of a ciliated^'circlet, to 

 which a second and afterwards others are 

 added, forming altogether the peculiar bran- 

 chial tissue. These cilia give rise to currents 

 in the digestive and subcutaneous cavities. 

 The young Ascidian can now carry on the 

 functions of respiration and digestion. 



At the surface of the skin, and around the 

 anal and branchial cavities there are apparent 

 longitudinal striae, which are evidently the rudi- 

 mentary muscular bands. The sphincters, also, 

 at the two extremities of the intestinal tube, 

 are brought into action. Some slight further 

 modifications only are now required to perfect 

 the development of the little Ascidia. 



ANATOMY OF CLAVELLINID.E. Clavellina. 

 At the superior extremity is the buccal 

 orifice, which is circular, looking directly 

 upwards (fig. 768. c), and furnished with a 

 very thin, prominent, cylindrical margin ; to 

 the interior of which is attached a ring or 

 circlet of simple tentacular filaments, about 

 thirty in number, of which about ten are 

 long enough to reach to the centre of the 

 cavity of the tube, whilst the others are 

 very short, and situated between the first. 

 Near this first orifice, and toward the supe- 

 rior part of the dorsal aspect of the thorax, is 

 the second or anal orifice, which is also cir- 

 cular, with an entire rim. The external 

 tunic or tegumentary membrane (test) of the 

 body is thin, but subcartilaginous. It adheres 

 feebly to the other parts, except around the 

 two orifices. At its base, or inferior extre- 

 mity, there are a variable number of raciici- 

 form prolongations, which serve to fix the 

 animal, and some of which often bear at in- 

 tervals little pyriform tubercles, which be- 

 come developed into new individuals. 



