446 ZOOLOGY. 



and Wasps, Gall-flies and Ichneumon-flies, whose larvae 

 are parasitic in caterpillars. 



PHYLUM CHORDATA. 



Animals which possess temporarily or throughout life 

 the following three characters : (1) a dorsal tubular nerve 

 cord ; (2) pharyngeal gill-slits ; (3) a iiotochord. The 

 more primitive forms are distinguished by never develop- 

 ing any skull or vertebrae, nor any complicated brain like 

 that of the higher forms; they may be termed therefore 

 the Prochordata. 



Of the Prochordata two classes are included in the 

 syllabus, the Urochorda and the Cephalochorda. 



CLASS: UROCHORDA. 



These are the animals commonly known as Ascidians or 

 sea- squirts. The great majority of them are sessile in the 

 adult condition. In the larval stage of all and through- 

 out life in the Larvacea there is a notochord in the tail 

 which does not extend into the anterior part of the body. 

 There are at first only two gill- slits, which become enclosed 

 in a secondary cavity or depression into which the anus 

 opens ; this cavity is the atrium, though not exactly 

 homologous with the atrium of Amphioxus. The gill- slits 

 become subdivided into numerous small openings. The 

 atrial opening is dorsal and near the mouth. Thus the 

 usual form of the adult is that of an irregular somewhat 

 cylindrical sac, attached at one end, with two openings, 

 mouth and atrial opening, at the other. The cilia of the 

 pharynx maintain a constant current of water, entering at 

 the mouth and passing out at the atrial aperture. A 

 ciliated and glandular endostyle is present along the 

 ventral median line of the pharynx as in Amphioxus, and 

 the epipharyngeal groove is represented by a dorsal series 

 of projections called languets. Another very characteristic 

 feature is that the epidermis secretes a thick gelatinous 

 "test" into which extends an outgrowth from the hinder 

 part of the body, and froin this outgrowth the test receives 

 cells and blood-vessels ; the test contains cellulose. Many 



