Miscellaneous. 165 



form varies according to the position which they occupy in the masses : 

 the individuals placed in the centre become elongated, in order that 

 they may not be covered up by their neighbours ; frequently there 

 is even developed at the side opposite the siphons a peduncle, which 

 may attain a length of two centimetres. 



As might be foreseen, this Ascidian, being fixed, produces a 

 urodclous larva, the caudal appendage of which even attains a con- 

 siderable development. The terminal portion of the tail presents 

 rays of firmer texture than the rest of the membrane, which I 

 cannot better compare to any thing than the natatory rays of the 

 embryos of fishes. 



But this arrangement is not strongly marked in the larva of our 

 Mohjula ; and I have only cited this species in the first place because 

 it seemed to me remarkable to find in the same genus, side by side 

 with anurous larvae, others presenting a caudal appendage so highly 

 organized. 



A much higher degree of complication is observed in the embrj'os 

 of certain Cynthicn, and especially in the species belonging to the 

 group Stijela of Savigny. I have more particularly investigated a 

 little Ascidian (4-6 millims. in height), the anatomical stnicture of 

 which closely approaches that of tSti/ela, but which, in accordance 

 with the antiquated and perfectly artificial nomenclature still adopted 

 in recent works on the Ascidia, must be placed in a very difi'erent 

 group. In fact this species, which I shall call Poh/styela Lemirri, 

 is a composite, or at least a social, Ascidian. The individuals {hlasto- 

 zdites) forming each colony (cormus) are arranged upon a common 

 basal lamina, like the coral-polyps of the genus Sympodium. Pohj- 

 styela approaches the Synascidia by other morphological and even 

 by physiological characters : the embrj'os issue fully formed from 

 the maternal organism, within which the incubation has been eff'ected. 



The caudal appendage of the embryo of Polystyela presents 

 throughout its length perfectly developed and very regularly arranged 

 rays. These rays, which are perpendicular to the dorsal cord in the 

 first part of the tail, become gradually more inclined towards the 

 axis in proportion as they approach the extremity. This extremity 

 itself also surprisingly resembles that of a yonng fish ; a drawing 

 representing it could hardly be distinguished from that which might 

 be made of the embryo of Macropodus viridis ornatus, for example. 



In the first portion of the tail, and nearly to the posterior third 

 of the dorsal cord, the natatory rays, which are very slightly inclined 

 to this cord, present a cartilaginous base of granular aspect, occupy- 

 ing about one third of the membranous limb of the appendage. 

 We have consequently at this point a structure nearly identical with 

 that which is observed in the caudal appendage of the young salmon 

 about the tenth day of its development, namely cartilaginous 

 supports resting or not resting upon the cord, and terminated by 

 rays sustaining a delicate membrane. I content myself at present 

 by indicating these curious facts, proposing some day to revert to 

 the signification which mav be ascribed to them. — Comptes Jieyidus, 

 June 29, 1874, pp. 1800-180:}. 



