434 



SCIENCE. 



[N. S. Vol. V. No. 115. 



^he inner vesicle, but near the point where 

 the digestive tract is growing out. 



It is quite probable that free cells of the 

 blood also take part in the formation of all 

 these organs, as appearances strongly indi- 

 cate such an occurrence, but these cells 

 themselves are derived from the inner 

 vesicle, which is clearly seen to give them 

 off into the body space, especially at very 

 early stages. 



The ectoderm, therefore, is not actively 

 concerned in the bud development, but the 

 duty of providing the material for the for- 

 mation of all the internal organs devolves 

 solely upon the inner or ' endodermal ' 

 vesicle. 



Notes on the Structure and Development of the 

 Type of a New Family of so-called Social 

 Ascidians from the Coast of Califoinia. W. 



E. RiTTEE. 



In its superficial characters the new form 

 closely resembles Clavelina. Studied in de- 

 tail, however, its affinities are found to be 

 much closer with the Polyclinidce, e. g., with 

 the genus Amaroucium, than with Clavelina. 

 The acidiozoids are whollj' distinct from 

 one another, excepting for their attachment 

 to a common basal stolon, as in Clavelina, 

 and in form, size and color they closely re- 

 semble the zooids of some species of this 

 genus, e. g., C. savigniana M. Edw. 



They are Hute-shaped, the attachment 

 being at the small end. Their average length 

 is about 3 cm. The colonies usually con- 

 tain many zooids closely crowded together, 

 as in Clavelina. But beyond this the dis- 

 tinctively clavelinian characters cease. The 

 general features of the individual zooids are 

 distinctly those of the Polyclinidce. The body 

 is divided into three well-defined regions: 

 viz, the thorax, containing the branchial 

 sac ; the abdomen, composed mainly of the 

 intestine ; and the post-abdomen, contain- 

 ing the gonads and the heart. 



In Clavelina, on the other hand, the 



gonads are situated within the intestinal 

 loop, and the heart along side of it — in other 

 words, Clavelina has no post-abdomen. 



ISTow it will be noted that the Polyclinidce 

 are entirely typicical ascidi^e compositse ; 

 ^. e., not only does repi-oduction by gem- 

 mation take place, but the blastozoids 

 thus produced become closely crowded to- 

 gether and all wholly embedded in a com- 

 mon testicular mass. 



This brief comparison will suffice to call 

 attention to the fact, which becomes much 

 more striking when the comparison is 

 carried out in detail, that in the new form 

 we have an ascidian which in the relation of 

 the blastozoides to one another in the colony is 

 strictly a so-called social ascidian, while in the 

 structure of the individual zooids it is as strictly 

 a compound ascidian. 



Another illustration is thus produced of 

 the artificialty of a classification of the tuni- 

 cata which attempts to base primary sub- 

 divisions on the condition of the blas- 

 tozoides of the colony as regards a common 

 test- mass. 



The characters which prevent the form 

 from being admitted to the family Polycli- 

 nidce, and make necessary the establishment 

 of a new one for it, are founded in the struc- 

 ture of the gonads and the oviduct ; the 

 relations of the epicardiac tubes; and in the 

 arrangement of the branchial tentacles. 



The study of the embryology is still quite 

 incomplete. The embrj'os are developed in 

 a long expanded proximal portion of the 

 oviduct which may properly be called a 

 uterus. About a dozen embryos are found 

 in each uterus, these being placed in a single 

 row, usually with the oldest farthest for- 

 ward and the youngest nearest the ovary, 

 or farthest back. 



The larval stage is much abridged, the 

 metamorphosis of the nervous system being 

 nearly complete before the larva leaves the 

 parent. It is doubtful if there is any free- 

 swimming tadpole stage at all. 



