656 



CHORDATA 



/ 



line of the pharynx, the secretions of which serve to entangle the minute 

 organisms which form the food of the animals, and the action of whose 

 cilia carries these forward to the ciliated peripharyngeal band (Fig. 

 998,5) which surrounds the anterior end of the phaiynx. This band 

 passes the food dorsally to the dorsal lamina (9), a ciliated band in the 



mid-dorsal line, along which it is car- 

 ried to the opening of the oesophagus 

 at the inner end of the pharynx. The 

 short oesophagus joins the pharynx 

 with the stomach (15), from which the 

 intestine passes to the anal opening 

 into the cloacal space. 



The heart (12) is ventral in posi- 

 tion and lies in a pericardial space, 

 which alone represents the only part 

 of the coelom in the adult. The con- 

 tractions of the heart change their 

 direction regularly, passing first for a 

 time from one end of the heart to the 

 other and then after a moment of rest 

 in the opposite direction, a peculiarity 

 which occurs only in tunicates. The 

 blood circulates mainly in spaces in the 

 mantle and the tunic and in the walls 

 of the branchial sac. The nervous sys- 

 tem consists, first, of a simjjle dorsal ganglion (4) which lies in the 

 mantle near the mouth, and, second, of nerves which radiate from it. 

 Simple eyes and tactile organs are present in many species. Tunicates 

 are mostly hermaphroditic, the gonads being 

 either single or paired and the genital ducts 

 opening into the cloacal chamber. Asexual 

 reproduction is very general, and in many 

 species may lead to the formation of colo- 

 nies. The adult structure of the Larvacea 

 as well as the embryonic and larval structure 

 of the Ascidiacea present many points of 

 resemblance to Amphioxus and the lower 

 vertebrates, indicating a relationship between 

 these animals and tunicates. This resem- 

 blance consists in the main in the presence of 

 a tubular, dorsal central nervous system, a notochord, a ventral heart, 

 and the pharyngeal respiration. 



Fig. 998 — Diagram of a simple 

 ascidian (Boas). 1, mouth ; 2, cloa- 

 cal opening ; 3, subneural gland ; 4, 

 principal ganglion ; 5, peribranchial 

 ridge ; 6. endostyle ; 7, tunic ; 8, 

 pharynx ; 9, dorsal lamina ; 10, 

 peribranchial space ; 11, mantle ; 

 12, heart ; 13, anus ; 14, intes- 

 tine ; 15, stomach; 16, testis; 17, 

 ovary. 



Fig. 999 — Diagram of a 

 cross section of an ascidian 

 (Boas). Explanations as in 

 Fig 998. 



