70 



ASCIDIANS 



CHAP. 



frequent round the British coasts, our commonest species being 

 probably 0. dioica, Fol, and F. furcata, Moss. Young specimens 

 appear in the plankton about February and March, and larger 

 forms are as a rule found later in the summer. Several instances 

 have been recorded of swarms of especially large forms, provided 



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Fig. 32. — Diagram oi Fritillaria seen from the right side to show the elongated body, 

 the hood, and the relative positions of anus, atrial opening, and gonads. (Compare 

 "with Oiko2}leura, Fig. 30.) ci. Anus ; at^ opening of atrial tube ; 6r.5, branchial sac ; 

 end, endostyle ; ht, heart ; m, mouth ; n.ch, uotochord ; n.g, nerve-ganglion ; oes, 

 oesophagus ; ov, ovary ; sg, stigma ; sp, testis ; st, stomach. 



with massive tests (the " house "), having appeared suddenly on 

 our coast in such abundance as to form an important element in 

 the surface life of the sea. 



Order II. Ascidiacea (Ascidians). 



Fixed or free-swimming Simple or Compound Ascidians, which 

 in the adult are never provided with a locomotory appendage or 

 tail, and have no trace of a notochord. The free-swimming 



O 



forms are colonies, the Simple Ascidians being always sedentary 

 and usually fixed. The test is permanent and well developed, 

 and becomes organised by the immigration of cells from the body ; 

 as a rule it increases in size with the age of the individual. The 

 branchial sac is large and well developed. Its walls are perforated 

 by numerous slits (stigmata) opening into the peribranchial 

 cavity, which communicates with the exterior by the single atrial 

 aperture. Many of the Ascidiacea, both fixed and free, reproduce 

 by gemmation to form colonies, and in most of them the sexually 

 produced embryo develops into a tailed larva. 



The Ascidiacea includes three groups, the Simple Ascidians, the 

 Compound Ascidians, and the free-swimming colonial Pyrosoma, 

 which in some respects connects this Order with the Thaliacea. 



