138 CHORD AT A. 



Subphylum B. Urochorda (Tunicata). Ascidians. 



Marine hermaphrodites, usually of sac- or cask-shaped form. An ex- 

 ternal test of gelatinous or cartilaginous nature. Oral and atrial aper- 

 tures. The first section of the gut is a pharynx perforated by gill-slits 

 (branchial sac). Heart tubular and reversible. Asexual reproduction 

 common, colonies being formed by budding. 



In simple instances, e.g. Ascidia, the development shows great har- 

 mony with that of Amphioxus. After complete cleavage a gastrula is 

 formed by embolic invagination. From the ectoderm a dorsal neural 

 tube is developed, while a notochord (urochord) arises from the posterior 

 part of the archenteron. The tailed larva exhibits metameric segmenta- 

 tion, but later on becomes an unsegmented animal by loss of the tail 

 (with notochord), and reduction of the neural tube. In the Thaliacea 

 there is a true alternation of generations (sexual and asexual generations 

 alternate). 



Order 1. Larvacea (Perennichordata, Copelatse). Small free-swimming 

 larva-like Ascidians ; the swimming tail persists, as also the nerve-cord, 

 myotomes, and notochord (urochord). The anus and the two gill-slits 

 open directly to the exterior. A temporary gelatinous test secreted from 

 time to time by the epidermis. Appendicularia, Fritillaria. 



Order 2. Ascidice (Tethyodea, Caducichordata in part), sea squirts. A 

 well-developed cartilaginous test and large pharynx perforated by 

 numerous secondary gill-slits. Mostly sedentary. 



Ascidise simplices, oral and atrial apertures approximated: 

 Clavellina ; Ascidia mammillata ; Oiona intestinalis. Octacnemus 

 bythius, a deep-sea form. 



Ascidise compositse, colonial forms with a common test ; indi- 

 viduals arranged in systems, each of which has a central cloacal cavity : 

 Botryllus. 



Ascidise salpiformes; free swimming colonies in the form of a 

 hollow cylinder closed at one end ; the embryo develops into an asexual 

 cyathozooid, which produces four sexual ascidiozooids by budding ; 

 these constitute a new colony which continually enlarges by gemmation. 

 Pyrosoma, phosphorescent. 



Order 3. Thaliacea (Caducichordata in part), salps. Pelagic transparent 

 ascidians. Oral and atrial apertures at opposite extremities. Develop- 

 ment with alternation of generations, sexual individuals united in a 

 chain. Salpa, wall of pharynx reduced to a dorsal band (gill), S. de- 

 mocratica with S. mucronata as the chain-form. The egg develops 

 to an embryo, which nourishes itself by a placenta in the brood-sac of 

 the parent, and is born as a solitary asexual salp (nurse). Doliolum 

 with pharynx perforated by two transverse rows of gill-clefts and com- 

 plicated alternation of generations. 



