REPORT ON THE THNICATA. 
309 
the present family and the Didemnidm, where the vas deferens always commences by 
coiling spirally around the single large testis. 
Pyloric gemmation, which is sometimes met with in the Didemnidm, is found typically 
in the Diplosomidse, where it produces in the Ascidio 20 oids the curious double-bodied 
appearance (see PI. XLII. figs. 7, 8) first noticed by Macdonald in Diplosoma rayneri. 
Each new Ascidiozooid produced by gemmation in the colony is formed by the union 
of two distinct buds, which either arise independently from the body of the parent, 
or separate very soon after their origin, the one becoming the thorax and the other the 
abdomen. The first process of gemmation, however, is so early that it commences in the 
embryo. Consequently the larva produces at least two Ascidiozooids in place of one, but 
aU the future Ascidiozooids of the colony have their bodies formed by two distinct buds ' 
in place of by one, as in the case of other Ascidise Composite. 
It has been shown conclusively by von Drasche that the three genera Diplosoma, 
Pseudodidemnum, and Astellium were distinguished by Giard uf)on quite insufficient 
grounds, so that the two latter names must be given up, leaving Diplosoma, Macdonald, 
as the only genus in the family. Jourdain * has recently changed Giard s Astellium into 
Brevistellium. Both names are unnecessary and must be abandoned. The suggestion 
made by von Drasche ^ that the species forming thicker colonies might with advantage 
be separated from those which produce mere incrusting films may possibly have to be 
carried out, but I believe that it is more practicable in the present state of our knowledge 
of the grouj) to divide the family into two genera, in the one of which the test has no 
calcareous spicules, while in the other spicules are present, and I would propose the 
name Diplosomoides for the latter group, leaving Diplosoma for the more typical 
members of the family, with soft transparent colonies. The species of Diplosomoides 
are obviously more nearly allied to the Didemnidse than are those of Diplosoma. 
Diplosomoides, n. gen. 
Colony usually thin and incrusting, sometimes only slightly attached. 
Test gelatinous, sometimes firm externally, more or less opaque and of a whitish 
colour. Calcareous spicules present in the superficial layer. 
The other characters of this genus are the same as those of the following one, 
Diplosoma, the single important distinguishing feature being the presence of spicules in 
the test. This, however, gives rise to opacity, and produces a whitish colour in the 
1 According to Jourdain, however {Comptes rendus, t. c., No. 24, p. 1512), the two buds arise as a single projec- 
tion, from the oesophageal region of the body, which very soon divides into two parts. 
2 Comptes rendus, t. c., No. 24, p. 1512. 3 j)ie Synascidien, p. 39. 
