70 ASCIDIANS CHAP. 
frequent round the British coasts, our commonest species being 
probably O. 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 
hood 
Fia. 82.—Diagram of 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 Oikopleura, Fig. 30.) a, Anus; at, opening of atrial tube ; 67.s, branchial sac ; 
end, endostyle ; ht, heart ; m, mouth; x.ch, notochord ; 7.g, nerve-ganglion ; ves, 
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 
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. 
