2qO MANUAL OF ZOOLOGY. . 



with valves. In consequence of this, the circulation in the 

 majority of Tunicaries is periodically reversed, the blood 

 being propelled in one direction for a certain number of con- 

 tractions, and being then driven for a like period in an oppo- 

 site direction ; " so that the two ends of the heart are alter- 

 nately arterial and venous." 



The nervous system consists of a single ganglion placed on 

 one side of the oral aperture, between it and the anus, in all 

 known Tunicata, except in the aberrant form Appendicular ia. 



The only organs of sense are pigment-spots, or ocelli, placed 

 between the oral tentacles, and an auditory capsule, sometimes 

 containing an otolith. These organs, however, do not appear 

 to be constantly present. 



With the exception of Doliolum and Appendiadaria, all the 

 Tunkata are hermaphrodite. The reproductive organs are 

 situated in the fold of the intestine, and their efferent duct 

 opens into the atrium. The embryo Tunicate is at first gene- 

 rally free, and is mostly shaped like the tadpole of a frog, 

 swimming by means of a long caudal appendage. In one 

 species {Molgula tubulosd) the larval form is destitute of a tail, 

 inactive, and amoeboid, and it almost immediately attaches 

 itself by means of little outward processes which it develops. 

 Lastly, in several instances the larval caudal appendage has 

 been shown to exhibit a cylindrical rod-like body, which has 

 been paralleled with the chorda dorsalis of Vertebrates. 



Amongst the Salpians a species of alternation of generations 

 has been observed. A solitary Salpian produces long chains 

 of embryos, which remain organically connected throughout 

 their entire life. Each individual of these associated speci- 

 mens produces solitary young, which are often very unlike their 

 parents, and these again give rise to the aggregated forms. 



The Tunicata are often spoken of as exhibiting three main 

 types of structure, which give origin to as many sections, 

 known respectively as the solitary, the social, and the compound 

 forms. In the " solitary" Tunicaries, the individuals, however 

 produced, remain entirely distinct, or, if not so primitively, 

 they become so. In the " social " Ascidians the organism 

 consists of a number of zooids, produced by gemmation and 

 permanently connected together by a vascular canal, or "stolon," 

 composed of a prolongation of the common tunic, through 

 . which the blood circulates. Finally, in the " compound" forms, 

 the zooids Ijecome aggregated into a common mass, their tests 

 being fused together, but there being no internal union. The 

 Botrylli, which are familiar examples of the compound Tuni- 

 cates, form semi-transparent masses, often of brilliant colours. 



