CHORDATA 235 



nerve ganglion. It envelops itself in a protective " tunic," 

 and becomes as we see it in Fig. 102. 



In one species, however, a diminutive one, named Ap- 

 pendicularia, which abounds in our seas, the tadpole form, 

 with very little modification, only that of loss of symmetry, 

 is permanent through life. 



On our rocky shores, in pools that do not drain dry, 

 fairly well down in tide range, the intending student will 

 find attached to the sides of stones, below their water line, 

 and on their under sides, clusters of the beautiful little 

 crystal bell, Clavellina, and in the months of May and June 

 he will easily find some in which the atrial chamber contains 

 a vast number of the tadpole larvae, which will, with a 

 little persuasion, readily emerge, and swim nimbly in a 

 dish of sea-water, affording an entertaining object of study. 

 All species, in proper season, offer the same facilities, but 

 clavellina, being transparent, shows if the larvae are present 

 without dissection. 



The tunica tes are roughly divided into three sections 

 viz. 



" Simple Ascidians," which are usually of large size, and 

 live isolated ; " Social Ascidians," in which the individuals, 

 while in one sense separate, are still joined into colonies 

 by a stem or " stolon," and the " Compound Ascidians," 

 those in which the individuals combine into large colonies, 

 usually in colonies of little families, and are enveloped in 

 one spreading, leathery tunic, common to all, and which 

 forms encrusting coatings to stones, etc. The " Starry 

 Botryllus " is one of these (see Figs. 100 and 101). 



Some tunicates, considerably modified in outward form, 

 float, either singly or in curiously arranged chains, in the 

 open sea (Salpa, Doliolum, etc.). 



The tunicates, both in number of species and number 

 of individuals, abound on our shores. The compound 

 forms being most numerous. 



