UROCHORDA. 445 



Both dorsal and ventral cardiac vessels give off branches to the test. The 

 blood has a clear plasma with nucleated and usually rounded corpuscles, 

 many of which are sometimes pigmented (opaque white, yellow, red, brown, 

 purple, blue). Specialised renal glands are not present (? the neural gland). 

 Many Ascidiidae, however, have clear vesicles in masses round the intestine 

 and in the body walls, containing concretions. And in the Molgulidae 

 there is a sac-like organ close to the pericardium, containing rounded con- 

 cretions, in which uric acid has been detected. A yellowish-green mass 

 usually coats the first part of the intestine in A. Compositae, and perhaps 

 corresponds to the clear vesicles of the Ascidiidae. 



All Urochorda are hermaphrodite, but the male and female organs 

 appear to become mature at different times. Sexual organs are absent, or 

 at least atrophied, in the nurse forms of Salpa and Doliolum, in the Cyatho- 

 zooid of Pyrosoma, and in certain generations of Botryllus. Both testes and 

 ovary in Larvacea are simple saccular organs, devoid of ducts. They lie 

 behind the stomach, and their contents are set free by dehiscence of the 

 body walls, and the organism then dies. In other Urochorda the organs 

 are either more or less saccular, as in Ascidia, or else branching tubes, and 

 their ducts either open close together or by a common aperture, as in 

 Doliolum, into the atrial cavity, and close to the anus. There is but a 

 single ovum in Pyrosoma and Salpa. In most Urochorda except the Lar- 

 vacea^ follicular cells surround the ovum, and are inclosed with it in a 

 membrane. The follicle cells sometimes give origin to a chorion enveloping 

 the ovum, and in some instances grow out into long external villiform 

 processes. Within the chorion appear in many instances a number of so- 

 called test-cells. The origin and fate of both the follicular and the test- 

 cells are involved in much difficulty (see lit. cited below). Impregnation 

 and sometimes development take place in the atrial cavity, in a special 

 incubatory pouch opening near the anus (some A. Compositae), or within 

 the ovary, as in Pyrosoma and Salpa. Segmentation is regular, except in 

 Pyrosoma, where it is meroblastic, a germinal disc being formed. There is 

 an invaginate gastrula, or in Molgula (? all species) a gastrula by over- 

 growth. In Salpa the developing embryo is nourished by a placenta 

 formed, in part at least, by follicle cells ; and certain cells gonoblasts 

 derived from the follicle cells have been stated to take the chief part in the 

 formation of the embryo itself (Salensky), but details vary in different 

 species. Except in Salpa, Pyrosoma^ and Molgula tubulosa, there is a larva 

 which resembles in many respects one of the Larvacea. It possesses a 

 straight swimming tail, supported by a notochordal rod, moved by lateral 

 muscles, and containing an extension of the nervous system. After a 

 certain period of free existence, the larva attaches itself by means of the 

 glutinous secretion of glands borne by three papillae developed at the 

 anterior end of the body. The papillae subsequently atrophy, and the 



