Ill ASCIDIAE SIMPLICES—-CLAVELINIDAE 71 
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. 55), 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. 35), 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 Fic. 33.—Colony of Clavelina lepadiformis 
nets : < (nat. size). 
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 :—Lcteinascidia, Herdman, with internal longitudinal bars 
in the branchial sac; Clavelina, Savigny, with a long body and 
intestine extending behind the branchial sac (Fig. 35); and 
