442 



GENERAL ZOOLOGY 



ically reversing the direction of its beat, driving the blood toward 

 the gills for a certain interval and then driving it in the opposite 

 direction to the viscera, etc. The central nervous system 

 develops from a tube and is entirely dorsal to the alimentary 

 canal. In the adult it is represented by a simple dorsal 



ganglion, located in the mantle near 

 the mouth. Close to it is the sub- 

 neural ganglion, thought by some to 

 be homologous with the hypophysis 

 of vertebrates. Tunicates are gener- 

 ally hermaphroditic. The tailed 

 larvae of the sessile tunicates show 

 unmistakable evidence of relationship 

 to Amphioxus and the lower verte- 

 brates. Some of these larval char- 

 acters are retained in the adult stage 

 by free-swimming forms. In some 

 colonial tunicates (Salpa) there is an 

 alternation of generations between a 

 solitary asexual form and a sexual 

 chain form. All tunicates are marine. 

 Examples : Oikopleura flabellum, 

 pelagic, retains larval characters; 

 Ciona intestinalis, the sea squirt, 

 . about 7 cm. in length, a common New 

 England form: Salpa democratica, 

 about 2 cm. in length, a free-swim- 

 ming form; Amaroucium stellatum, 

 known as sea pork, a colonial tuni- 

 cate common in the Cape Cod 

 region. 

 SUBPHYLUM 3. LEPTOCARDIA. 

 The body is fish-shaped but lacks a 

 clearly defined head (Fig. 265). The 

 mouth, encircled by numerous cirri, lacks jaws and is located 

 slightly behind and under the anterior end of the body. The 

 pharynx, into which the mouth opens, is provided with numerous 

 gill slits that connect externally with a branchial chamber, from 

 which water taken in through the mouth leaves by the atriopore. 

 The atriopore is a median, ventral opening at the posterior end 



Fig. 264. — Diagram of a 

 Tunicate, one-half of the tunic 

 removed, a, anus; at, at- 

 rium; c, cloaca; e, excurrent 

 siphon; g, gonad whose duct 

 following the intestine opens 

 into the cloaca; h, heart; i, 

 incurrent siphon; in, intestine; 

 ng, nerve ganglion; o, esopha- 

 gus; p, pharynx; pv, perivis- 

 ceral cavity; s, stomach; t, 

 tunic. (Based on Leuckart- 

 Nitsche wall chart.) 



