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SUB-PHYLUM TUNICATA OR UROCHORDA. 



What are Tunicates ? 



Tunicates or Ascidians have the body enclosed in a tunic 

 or test of cellulose ; they also have a large pharynx, usually 

 with numerous gill-slits, and more or less completely sur- 

 rounded by the atrium, a large space between the pharynx 

 and the body wall. 



Metamorphosis (retrogressive) is usually a feature of their 

 development. The tadpole-like larva has a tubular and 

 dorsal central nervous system, a brain-derived eye, and a 

 notochord in the tail region ; but these characteristic 

 Vertebrate structures generally disappear during transition, 

 and are not commonly present in the adult. 



Some Tunicates are solitary, others form colonies ; and 

 they are either sedentary, fixed to one spot, or free and 

 palagic. Propagation by budding is a frequent mode of 

 multiplication. 



Give a general Account of (I) the Reproduction, and (2) the 

 Development of the Tunicata. 



1. Tunicates are, with few exceptions, hermaphrodite ; 

 the gonads mature at different times (i.e., dichogamy. See 

 page 12, Part I.), and usually the ova ripen before the 

 sperms (i.e., protogyny, a rare phenomenon among animals). 

 Asexual reproduction by budding is also frequent, and re- 

 sults in clusters (e.g., Clavellina) or in close colonies (e.g., 

 Botryllus). The buds arise from a common stolon (except 

 Botryllus). In several instances (e.g., Salpa, Doliolum) 

 both modes of reproduction occur in the course of the life- 

 history, which shows alternation of generations or meta- 

 genesis (See page 13, Part I.) ; the embryo, or the larva, 

 becomes an asexual form, which by budding produces 

 sexual forms, and these beget a new asexual generation. 

 The asexual form of Doliolum produces buds of three 

 different kinds (polymorphism), namely, sexual zooids and 

 their " foster-mothers," and nutritive zooids. 



2. The fertilised egg is developed either outside the body 

 (i.e., in the sea) or within the atrium, or in special brood 

 pouches of the atrium (e.g., Compound Ascidians) ; and 

 generally a free-swimming* tailed larva is formed. The 

 larva has a large pharynx, with few gill-slits and an endostyle ; 

 and the short intestine, bent upwards, ends in a dorsal anus 

 in front of the tail. The atrium is developed from two 

 invaginations of ectoderm which gradually extend around 

 the pharynx. The tubular nervous system consists of 



