TUNICATA 



1225 



of the test is carried on by imbibition only ; 

 and M. Milne-Edwards points out the fact of 

 the independence of these two portions of the 

 body of these Ascidians daring the early 

 periods of their life, as worthy the considera- 

 tion of physiologists ; and he adds, that proba- 

 bly this kind of vitality of the integument of 

 the larval Ascidians has some analogy with 

 what obtains in Sponges, and may, perhaps, 

 throw some light on the peculiar existence of 

 the basilar portion of the Sertularke and 

 other Polyps, that continue to live for some 

 time after the loss of the soft parts that are 

 generally, but wrongly, regarded as constitut- 

 ing the entire animal. 



From observations made by the same natu- 

 ralist on the development of the ova and 

 larvae of other species of the Polyclinina, and 

 of the Didemnina and ClavellincE, it appears 

 that very similar modifications take place ; 

 the time occupied in the development being 

 of course variable according to specific and 

 external conditions. The larvae of Clave/Una 

 have the internal tunic strongly lobed in front, 

 very tumid behind, and destitute of the pecu- 

 liar appendages observed in the PolycKnina. 

 These appendages exist in the Didemnina, but 

 are very short ; and at their base are seen a 

 row of pyriform lobules, which might easily be 

 taken for the germs of young individuals, but 

 all of which really belong to one individual. 



The observations made by MM. Lowig and 

 Kolliker on the embryogeny of some of these 

 animals are generally in accordance with those 

 of M. Milne-Edwards, but on some points, 

 and especially with regard to the develop- 

 ment of the Botrylli, there exists considerable 

 discordance of opinion. The points involved 

 being not only of interest, but of no slight im- 

 portance, it is necessary to give in detail some 

 of the most important observations recorded by 

 the above-named naturalists. 



In Botryllus violaceus, B. aureus, Apli- 

 dium gibbulosiim, and Amaroucium Nordmanni, 

 MM. Lowig and Kolliker observed, in the 

 first stages of development, a division of the 

 yolk similar to that taking place in the eggs 

 of frogs, and described also by M. Milne-Ed- 

 wards, and established also in the case of the 

 simple Ascidians by Van Beneden. This divi- 

 sion takes place as in the intestinal worms ; 

 that is to say, the simple nuclei, contained in 

 the globules, which, as everywhere, are only 

 aggregations of granules, always become double 

 before the globules are divided in two. As 

 soon as the division has arrived at a certain 

 degree, the spherical form of the mass of glo- 

 bules becomes elongated, and takes more and 

 more the form of an embryo, its tail making a 

 semicircle about its body. The tail is distinct 

 before any other part, and this, according to 

 these observers, is evidently formed, not by an 

 elongation of the embryo, but by the separa- 

 tion of a portion of the globules from the sur- 

 face. In Amaroucium and Ajj/idium, at the same 

 time as the exterior form of the embryo, and 

 at a very early period, the two ocular points, 

 remarked by Milne-Edwards, make their ap- 

 pearance; presenting as yet no trace of an 



envelope or of anterior processes. At a later 

 period only is there formed around the em- 

 bryo a transparent, colourless border, which, 

 in Amaroucium and Aplidium, increasing more 

 and more, especially at the thick end of the 

 body, appears as a very strong lamina, but 

 remains without structure all the time the 

 embryos are contained in the membranes of 

 the egg. Cotemporaneously with the forma- 

 tion of the envelopes, the embryos themselves 

 commence also to undergo a series of changes. 

 Firstly, towards the anterior extremity, there 

 appear the three appendages of peculiar form ; 

 secondly, the yellowish substance in the in- 

 terior of the body separates into two laminae ; 

 the one, external, remains diaphanous ; the 

 other, internal, becomes opaque, and divides, 

 in the Botrylli, into eight conical corpuscles, 

 which surround a somewhat large, round, 

 perforated nipple ; lastly, a great number of 

 structural modifications take place. The 

 mature embryos of B. aureus are formed of a 

 spherical body, 0-28'" broad and 0-38'" long, 

 possessing an orifice surrounded anteriorly 

 with three lobules, and posteriorly bearing a 

 thin, tapering tail, 0'72'" long. These em- 

 bryos present exteriorly the thin, transparent, 

 structureless layer (tegumentary layer) before 

 referred to, from which almost solely the lobes, 

 or lanceolate appendages of the head, are 

 formed, and which terminate at the opposite 

 extremity in a prolongation exceeding in length 

 those parts of the tail enclosed within it. In- 

 terior to this, in the anterior part of the bod}', 

 is a second delicate envelope, formed of cells 

 either round or changed into fibres, which 

 does not enter into the composition of the 

 lobes of the head, but encases the mammil- 

 lated prominence before described, and also 

 the eight spherical bodies surrounding it, and is 

 attached at one end to the edge of the nipple, 

 and at the other to the interior part of the 

 tail. 



The internal substance, constituting the 

 chief mass of the embryos, is, according to 

 MM. Lowig and Kolliker, evidently a group 

 of individuals, as M. Sars, who discovered 

 those curious embryos of the Botrylli, has 

 already shown. The eight spherical corpuscles, 

 united at their bases, and provided with a 

 kind of common stem, are so many individuals, 

 and the prominent nipple situated in their 

 centre represents the common excretory tube. 

 This salient tube at its extremity has three 

 lobules, which project into the base of the 

 lobes of the exterior envelope : from its base 

 three filaments (nerves ?) arise, which passing 

 upwards bifurcate each into two, one of 

 which terminates at the orifice of the tube ; 

 the second reaches the summits of the lo- 

 bules, and, passing beyond them, spread into 

 five or six branches, extending almost to the 

 edge of the lobes of the exterior envelope. 

 In the eight embryos no orifice is perceptible, 

 nor any other organs, except some canals 

 (intestine) indistinctly rolled up. Their mi- 

 croscopic elements, however, are very dis- 

 tinct ; namely, various-sized nucleated cells, 

 filled with pale red granules and fibres in 



