TUNICATA. 



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represented merely by the membrane that 

 surrounds the egg, and which, as before re- 

 marked, may be compared to the calyx of 

 birds. The testicle, on the contrary, is well 

 developed, but, not increasing in bulk in 

 proportion to the growth of the young Salpa, 

 it only acquires its greatest development in 

 nearly adult animals. It is always found in 

 the neighbourhood of the intestine, but its 

 position varies in different species. Some- 

 times occupying the centre of the visceral 

 mass or nucleus, formed by the intestinal loop 

 and its appendages, its presence is evident 

 only in proportion as it raises those parts ; 

 sometimes showing itself quite exposed, 

 it more or less covers a large portion 

 of the digestive apparatus. This organ in 

 the aggregate form of Salpa pinnata, S. pro- 

 boscidalis, &c. is spindle-shaped, and rests 

 against the intestine, and has been taken for 

 the liver by Cuvier, Chamisso, and Meyen. 

 The testicle is composed of a greater or less 

 number of ramified canals, the last ramifica- 

 tions of which end in culs-de-sac. All these 

 canals end in a principal duct, which, passing 

 along the terminal portion of the intestine, 

 opens at the side of the anus in the great 

 natatory or respiratory cavity traversing the 

 body of the Salpa. 



The testicle is very much slower in its de- 

 velopment than the egg organ in these ani- 

 mals ; and, as young aggregate Salpians are 

 met with in which a fecundated egg and 

 undeveloped testicle are co-existent, these 

 individuals must have had connection with 

 others in which the development of the 

 latter organ was further advanced ; the sperm 

 that fecundated the eggs being supplied by 

 another group of animals. The maturity of 

 the sperm does not coincide with that of the 

 egg, yet, as the two sexual organs are present 

 in the same body, these animals are herma- 

 phrodite, although probably not self-impreg- 

 nating. They have two generative functions 

 to perform : the one to produce a new being, 

 the other to fecundate an ulterior genera- 

 tion of animals similar in all respects to 

 themselves. The " aggregate" Salpians pro- 

 bably perish soon after they have given birth 

 to their "isolate" offspring. 



The placentn, or attachment of the fvetal "iso- 

 late" in the " aggregate" Salpce. The embryos 

 of the Salpa;, undergoing all the phases of de- 

 velopment within their mother, adhere to her by 

 the aid of an organ, the use of which is to 

 furnish them with the necessary nutritive ele- 

 ments. These elements being derived from the 

 blood of the mother, the vessels of the latter 

 enter largely into the composition of this organ. 

 Its structure and form vary much according 

 to the mode of propagation peculiar to each 

 generation. The propagation in the "iso- 

 lated" Salpians is by gemmation, and here 

 the organ in question is the proliferous stolon. 

 In the " aggregate " Salpian, on the other 

 hand, this organ is represented by a body 

 generally round, situated on the inferior 

 surface of their single foetus, and fixed to the 



internal wall of the maternal cavity, precisely 

 in the position occupied by the egg, and to 

 this organ many naturalists have with reason 

 attributed the functions of a placenta. This 

 olacenta is ordinarily situated in the external 

 tunic of the foetus; its structure is far from 

 being well known. In appearance it is a soft, 

 whitish, or brownish pulp, traversed by nu- 

 merous vessels ; it is attached by a very short 

 pedicle, which is formed by a prolongation 

 proceeding from the inner tunic, and envelop- 

 ing this organ. The vessels distributed in the 

 interior of the placenta communicate with 

 four trunks, two of which communicate with 

 the vascular system of the foetus, and the 

 other two with that of the mother. Those of 

 the foetus, in descending towards the placenta, 

 traverse the peduncle ; the one conducting 

 the blood to this organ, and the other re- 

 turning it to the foetus. The two maternal 

 trunks have analogous functions ; they ter- 

 minate at the point where the placenta is 

 attached to the mother. Each of these foetal 

 and maternal trunks alternately act as arteries 

 and veins. This alternation is due to the 

 periodic changes that determine the contrac- 

 tions of the heart in Tunicates generally. The 

 vessels of the f.etus and of the mother are 

 not in direct communication ; but, as in 

 Mammalia, they are merely contiguous. M. 

 Krohn's microscopical observations also tend 

 to prove that the maternal blood never com- 

 mingles with the foetal blood. The blood-cor- 

 puscles of the foetus are distinguishable from 

 those of the mother by their less size and 

 by the more constant regularity of their forms. 



The development of the placenta com- 

 mences at any early stage of enbryonic life, 

 and in its progress corresponds to that of 

 the foetus. Ultimately the placenta becomes 

 detached from the maternal tissues, and is 

 carried away by the young born animal. It 

 remains for a long time in connection with the 

 young animal, but decreases rapidly in size, and 

 ultimately disappears before the full growth 

 of the animal is perfected. 



Eleoblast of the foetal Salpce. There is 

 another organ belonging to the foetus, men- 

 tioned by authors, especially Chamisso, Me} en, 

 and Krohn, which is a round whitish body, 

 lodged, like the placenta, in the external 

 tunic of the foetus. Its use is entirely un- 

 known. It appears to be composed of a 

 multitude of fascicles or lamellae, that, by 

 their intercrossing with one another, cir- 

 cumscribe cellular cavities, filled with a per- 

 fectly clear oily liquid, composed chiefly of 

 globules. The fascicles or lamellae are tra- 

 versed by numerous vessels opening into 

 two trunks, which apparently form the at- 

 tachment between this organ and the visceral 

 nucleus. Meyen thought it to be the vitelline 

 sac of the foetus ; but, according to M. Krohn, 

 this opinion is inadmissible, because the 

 " aggregate " embryos, which we know are 

 not produced from eggs, are provided with it. 

 This body, M. Krohn terms the "eleoblast." 

 During the incubation of the foetus, and after 



4 K 2 



