EEPOET ON THE TUNICATA. 
199 
than the posterior, and being perfectly circular (PL XXV. fig. 11 , st.). The stomach is 
thin walled and has no folds. In this respect it differs from the stomach of Amaroucium 
Icevigatum, while in other respects the two alimentary canals are rather similar. The 
posterior end of the stomach tapers rapidly into the first portion of the intestine, which 
is a narrow tube running directly backwards. This is separated by a constriction from 
the second part of the intestine, which is wude and thick waUed (PL XXV. fig. 11, L) 
and turns abruptly towards the ventral side, and then anteriorly to open into the rectum. 
The last portion of the alimentary canal is a long thick-walled tube (PL XXV. fig. 11, r.) 
which runs forwards alongside the intestine, stomach, and oesophagus, crossing about the 
middle of its course so as to reach the dorsal side of the branchial sac. The rectum is 
much dilated at its commencement, and this part is separated from the intestine by a 
constriction or short narrow part of the tube. 
The post-abdomen is large, and is not separated by any external constriction from 
the abdomen (PI. XXV. fig. ll,p.ah.). The lower end of the post-abdomen is broad and 
rounded. 
This species and Amaroucium Icevigatum are certainly allied, notwithstanding their 
positions in distinct genera. 
Aplidium, Savigny. 
Aplidium, Savigny, M^moires, 1816. In part. 
Aplidium, Milne-Edwards, Observations, &c., 1842. 
Aplidium, Forbes, British Mollusca, voL i. p. 10, 1853. In part. 
Aplidium, Giard, Eecberches, &c., p. 635, 1872 (as a subgenus). 
Aplidium, Belle VaUe, Contribuzioni, &c., p. 34, 1877. In part. 
Aplidium, von Drascbe, Die Synascidieu, p. 25, 1883 (as a subgenus). 
Colony massive or lobed, not pedunculated. 
Systems compound and irregular. 
Ascidiozooids not much elongated ; branchial aperture six-lobed, atrial lobed or 
having a rudimentary languet, and placed far back. 
Test gelatinous or cartilaginous. 
Branchial Sac fairly well developed. 
Alimentary Canal of moderate size. Stomach-wall folded longitudinally. 
Post- Abdomen usually short. 
This genus, in the restricted sense in which it is now used, was established by Giard, 
and corresponds only to the first tribe of Savigny’s Aplidium, a section characterised by 
the shortness of the post-abdomen. Giard in his classification of the Polyclinidse used 
Aplidium in two senses (1) as a generic term opposed to Polyclinum, and including 
not only aU the forms that Savigny included but the genus Sidnyum in addition ; and 
