. The Cambridge natural history. Zoology. 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 Ivood. bcctl 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
. The Cambridge natural history. Zoology. 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 Ivood. bcctl 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 ; , branchial sac ; end, endostyle ; ht, heart; m, mouth ; , uotochord ; , 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.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
