<58 THE VOYAGE OF H.M.S. CHALLENGER. 



Body usually globular, attached or free, often incrusted with sand. Branchial 



aperture six-lobed, atrial four-lobed. 

 Test usually thin but tough, often having hairs on the outer surface. 

 Mantle thin and membranous ; musculature usually feeble ; consisting chiefly 



of long radiating bundles arising from the sphincters, and of short fusiform 



clumps of fibres scattered through the mantle. 

 Branchial Sac folded longitudinally ; stigmata more or less curved, coiled spirally 



in infundibula. 

 Tentacles compound. 



Alimentary Canal on the left side of the branchial sac. 

 Genitalia developed on both sides. 

 Renal Organ in the form of a crescentic sac placed in the centre of the right side 



of the mantle, and usually containing concretions. 



This genus has been so fully discussed recently by Lacaze-Duthiers in his great 

 monograph on the Molgulidee 1 that it seems superfluous to give a detailed account here 

 of the general characters. A few special points, however, require to be mentioned. As 

 is stated above, I have found it impossible in dealing with this collection to recognise 

 Lacaze-Duthiers' genus Anurella, on account of the absence in the adult animal of any 

 characters distinguishing it from Molgula. 



None of the distinguishing features of the genus Molgula can be derived from the 

 external appearance. Most of the species are globular and unattached, but on the other 

 hand some are elongated, and some quite irregular in shape ; some are attached, and some 

 are even shortly pedunculated. The condition of the test also furnishes no criterion. 

 Typically it is thin and membranous, but covered with sand-grains attached to long 

 hair-like processes. Molgula gigantea has its posterior half in this condition, but the 

 anterior part is perfectly smooth and has no incrusting sand, while Molgula pyriformis 

 is perfectly free on the entire surface both from adhering sand and hairs. 



The musculature of the mantle is characteristic for many of the species, but does not 

 hold for all. It is feebly developed on the whole, leaving the mantle transparent 

 (PI. V. fig. 9), and consists chiefly of (1), the sphincters round the apertures, which are 

 clearly defined and of moderate strength ; (2), a series of longitudinal bundles, which radiate 

 from the lower edge of each of the sphincters, and gradually die away as they recede 

 from the apertures ; and (3), of bundles of fibres scattered over the general surface of the 

 mantle. These last are partly the ordinary narrow greatly elongated bands found in other 

 groups, but they are chiefly a characteristic form found only in the Molgulidas, namely 

 short fusiform clumps consisting of from two to a dozen, but generally four to six, thick 

 short fibres closely united and tapering rapidly towards both ends so as to form a 



1 Les Ascidies Simples des cotes de France. 



