REPOET OX THE TUXICATA. 
307 
mantle. The nerve ganglion is conspicuous (PL XXXIII. fig. 15, n.g.) and is of irregu- 
larly rounded form. The aperture of the neural gland is distinct. In the specimen 
figured (PI. XXXIII. fig. 15) it seemed to open posteriorly to the periphaiyuigeal band. 
Family V. Diplosomidje. 
Colony forming a thin, incrusting layer, rarely thickened, never pedunculated. 
Systems irregular, usually inconspicuous. Common cloaca! apertures usually 
visible. 
Ascidiozooids divided into two distinct regions, thorax and abdomen. 
Test soft and gelatinous, usually transparent, rarely containing calcareous spicules. 
Vascular ectodermal appendages pro\dded with muscle fibres. 
Branchial Sac large, with four rows of stigmata. 
Dorsal Lamina represented by large languets. 
Alimentary Canal extending behind the branchial sac. Stomach smooth-walled. 
Reproductive Organs behind the intestinal loop or on its right side. Testis 
forming more than one mass. Vas deferens not spirally coiled. 
Gemmation pyloric. Larva gemmiparous. 
The first known member of this remarkable family was Diplosoma rayneri, described 
by Dr. J. D. Macdonald in 1859 ^ from a specimen obtained at Sydney, Australia. In 
the account of this new species and genus, Macdonald really gave the most essential 
characters of the family, but it was Giard who, in 1871,^ first formally defined the 
Diplosomidse. This latter author also added two new genera, Pseudodidemnum and 
Astellium, each containing a single species ; and in 1883 von Drasche ^ described several 
additional species belonging to the family, and pointed out the characters and relations of 
the group more fully and correctly than had previously been done. 
The family is evidently of small size, but seems to be widely spread in shallow water. 
It is more closely related to the Didemnidse than to any other group of the Ascidiae 
Compositae. Jourdain has recently proposed that these two families should be united 
under the name Oligosomidae. This is quite unnecessary, as the two groups are 
sufficiently separated by the structure of the reproductive organs. 
The colony in the Diplosomidae forms a very delicate incrusting layer, usually 
attached slightly by the greater part of the lower surface, and easily removed. It never 
becomes massive, and is always sessile. 
Both Giard and von Drasche describe the colony as formed of two thin layers of 
* Trans. Linn. Soc. Land., vol. xxxii. p. 373. ^ Recherches sur les Synascidies, Archives d. Zool. expdr., p. 654. 
^ Die Synascidien, &c., p. 38. 
^j3ur les Ascidies Composees de la ti’ibu des Diplosomidae, Comptes rendus, t. c., No. 24, p. 1512. 
