REPORT ON THE THNICATA, 
183 
there are generally from fifteen to twenty visible. The intestine runs posteriorly from 
the stomach for some distance (PL XXV. fig. 3, i.), and then turns abruptly towards the 
dorsal edge, and then anteriorly, so as to form a narrow loop. The rectum is wide and 
rather thin walled. 
The post-abdomen is nearly as wide as the abdomen, but is not very long. In 
transverse sections it shows the usual double median septum with a slit-like lumen and 
the two lateral cavities surrounded by masses of opaque granular cells. A few young 
ova were seen in the anterior part of one post-abdomen, but the reproductive organs 
seemed to be in an undeveloped condition in the other Ascidiozooids examined. 
Sidnyiim, Savigny. 
Sidrujum, Savigny, Meraoires, &c., addition, p. 2.38, 1816. 
Sidnyum, Gianl, Recherches, &c., Arcliives d. Zool. exp4r., voL i. p. 635, 1872. 
non Sydneum, Fleming, British Animals, p. 469, 1828. 
non Sidnyum, Alder, Trans. Tyneside Naturalists’ Field Club, vol. L p. 109, 1848. 
non Sidnyum, Forbes, British Mollusca, voL i. p. 13, and voL ii. p. 369, 1853. 
Colony massive, or formed of a number of lobes, one corresponding to each system. 
Systems compound. 
Ascidiozooids elongated and distinctly divided into regions ; branchial aperture 
six-lobed ; atrial aperture not j>rovided wdth a languet. 
Test gelatinous. 
Branchicd Sac well developed. 
Alimentary Canal forming a narrow loop. Stomach-wall irregularly thickened. 
Post-Abdomen long, and separated from the abdomen by a constriction. 
This genus was formed by Savigny for a species which he briefly described in an 
appendix to his “ Memoires,” published in 1816. The specimen, which he named Sidnyum 
turbinatum, had been found in British Seas by Leach and sent by him to Savigny. Its 
characteristics led Savigny to place it between Synoicum, Phipps, and his own genus 
Aplidium. He pointed out the similarity in the structure of the stomach-wall existing 
between Sidnyum and Synoicum, and described the post-abdomen as pedunculated, a 
point in which the two genera differ. Giard in his classification of the Polyclinidse 
recognised Sidnyum, but was apparently unable to decide whether it belonged to the 
genus Aplidium or the genus Polyclinum of his system. 
The new species which is described below, although agreeing in all essential characters 
with Savigny’s Sidnyum turbinatum, differs greatly in external appearance, and the two 
forms may therefore be readily distinguished. Sidnyum pallidum forms a globular 
mass with a broad and rounded upper surface (PI. XXV. fig. 4), while in Sidnyum 
