58 THE VOYAGE OF H.M.S. CHALLENGEE. 



simplest and least developed forms, and through them must be traced the passage to the 

 Ascidise Compositse. 



The relations between these families of Simple Ascidians may, according to our 

 present knowledge, be expressed serially thus : — 



MOLGULID^E — CYNTHIID.E — AsCIDIIDiE — ClAVELINID^E. 



Family Molgulid.e. 



Body usually free, sometimes fixed, rarely pedunculated. 



Test cartilaginous, coriaceous, or membranous, often covered with sand. Branchial 



aperture, six-lobed ; atrial aperture, four-lobed. 

 Branchial Sac longitudinally folded ; internal longitudinal bars not papillated ; 



stigmata more or less curved, usually arranged in spirals. 

 Tentacles always compound, usually much branched. 

 Intestine attached to the inner surface of the mantle on the left side. 

 Renal Sac present, upon the right side of the body. 

 Genitalia on the inner surface of the mantle, usually developed on both sides. 



This is the highest and most complex family of the Ascidise Simplices. It is closely 

 allied to the Cynthiidse, from which it was first distinguished by Lacaze-Duthiers in 1877. 1 



Heller, 2 who had previously divided the Ascidias Simplices into families, arranged 

 Molgtda and its allies under the Cynthiidas ; there can be no doubt, however, that the 

 two groups of genera should be considered as independent families. Lacaze-Duthiers 

 (loc. cit.) discusses this question at length, and after pointing out the resemblance between 

 Cynthia and Molgula he shows clearly the distinctions between the two forms, and 

 establishes and defines the family Molgulidse. 



The most constant and most generally useful characteristic is, as usual, to be found in 

 the configuration of the apertures. The branchial aperture has always six lobes, and the 

 atrial has always four. There are several other external characters, but none are so reliable 

 as these. The animal is usually free, or imbedded in mud or sand ; sometimes, however, 

 it is fixed like other Simple Ascidians, and a few of the newly discovered forms are 

 pedunculated. 



The outer surface of the test is usually covered by a thick coating of sand grains and 

 shell fragments adhering to long hair dike processes of the test. Some Molgulidae, how- 

 ever, have perfectly smooth tests with no adhering sand, while on the other hand some 

 of the Cynthiidaa {e.g., Polycarpa molgidoides) exhibit the delicate hairs and thick sandy 

 coating of a typical Molgula. 



The branchial sac, like that of the Cynthiidse, is longitudinally folded ; here, however, 



1 Les Ascidies Simjjles des cotes de France, Arch. Zool. expfr., t. vi. p. 457. 



2 Untersuchungen iiber die Tunicaten des adriatischen und Mittelmeeies, Abtli. iii. p. 1. 



