ASCIDIAE SIMPLICES CLAVELINIDAE 



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Sub-Order 1. Ascidiae Simplices. 



Fixed Ascidians, which are solitary, and very rarely reproduce 

 by gemmation ; if, as in a few cases, small colonies are formed, 

 the members are not buried in a common investing mass, but each 

 has a distinct test of its own. No strict line of demarcation can 

 be drawn between the Simple and Compound Ascidians ; and one 

 of the families of the former group, the Clavelinidae (the " Social " 

 Ascidians of Milne-Edwards), forms a transition from the typical 

 Simple forms which never reproduce by gemmation, to the Com- 

 pound forms which always do. Over 500 species of Ascidiae 

 Simplices are now known, but there are probably very many more 

 still undescribed. The sub-order may be divided into the follow- 

 ing families : — • 



Fam. 1. Clavelinidae. — Simple Ascidians which reproduce 

 by gemmation to form small colonies (Fig. 33), in which each 

 member, or ascidiozooid, has a distinct test, but all are connected 

 by a common blood-system, and by a prolongation of the " epicardiac 

 tubes" (see p. 83) from the 

 branchial sac. Buds are formed 

 on the stolons (Fig. 33), which 

 are vascular outgrowths from 

 the posterior end of the body, 

 containing prolongations from 

 the ectoderm, mesoderm, and 

 endoderm (the epicardium) of 

 the Ascidiozooid. Branchial sac 

 not folded; internal longitudinal 

 bars usually absent ; stigmata 

 straight ; tentacles simple. The 

 Clavelinidae are the simplest of 

 the Ascidiae Simplices. They are the forms that come nearest 

 to the Compound Ascidians, and are closely related to the 

 Distomatidae. They are probably the nearest .representatives 

 now existing of the ancestral forms from which both Simple and 

 Compound Ascidians are descended. 



This family contains amongst others the following three 

 genera : — Ucteinascidia, Herdman, with internal longitudinal bars 

 in the branchial sac ; Clavelina, Savigny, with a long body and 

 behind the branchial sac (Fig. 33); and 



Fig. 33. — Colony of Clavelina lepadiformis 

 (nat. size). 



intestine 



extending 



