xm PHYLUM CHORDATA 37 



number of which varies. The atrial cavity has grown round the 

 pharynx, and opens on the ^exterior by a single aperture only 

 (atr.). The heart and pericardial cavity have been formed. In 

 this tailed free-swimming stage the larva remains only a few hours ; 

 it soon becomes fixed by the adhesive papillae and begins to undergo 

 the retrogressive metamorphosis by which it attains the adult 

 condition. 



The chief changes involved in the retrogressive metamorphosis 

 (Fig. 745) are the increase in the number of pharyngeal stigmata, 

 the diminution, and eventually the complete disappearance, of the 

 tail with the contained notochord and caudal part of the nerve- 

 cord, the disappearance of the eye and the otocyst, the dwindling 

 of the central part of the nervous system to a single ganglion, and 

 the formation of the reproductive organs. Thus, from an active 

 free-swimming larva, with complex organs of special sense, and 

 provided with a notochord and well-developed nervous system, 

 there is a retrogression to the fixed inert adult, in which all the 

 parts indicative of affinities with the Vertebrata have become 

 aborted. The significance of these facts will be considered when 

 we come to discuss the general relationships of the Chordata. 



In some simple Ascidians, and in the composite forms in which 

 development takes place within the body of the parent, the meta- 

 morphosis may be considerably abbreviated, but there is always, 

 so far as known, a tailed larva, except in the genus Anwiella of 

 the MolgulidcB a family of the simple forms in which the tailed 

 stage is wanting and there is only an obscure endodermal rudiment 

 to represent the notochord. 



In Pyrosoma development is direct, without a tailed larval 

 stage, and takes place within the body of the parent. The ovum 

 contains a relatively large quantity of food-yolk, and the seg- 

 mentation is meroblastic. A process, developed at an early stage, 

 elongates to form the so-called stolon, which divides, by the 

 formation of constrictions, into four parts, each destined to give 

 rise to a zooid (tetrazooid). The primary zooid (cyathozooid) under- 

 goes atrophy, and at this stage the young colony, composed of four 

 tetrazooids with the remains of the cyathozooid enclosing a mass 

 of yolk the whole invested in a common cellulose test passes out 

 from the brood-pouch in which it was developed and reaches the 

 exterior through the cloaca of the parent colony. By a process of 

 budding from the four primary tetrazooids, the entire adult colony 

 is produced. 



The development of Doliolum is, in all essential respects, very 

 like that of the simple Ascidians. There is total segmentation, 

 followed by the formation of an embolic gastrula ; the larva (Fig. 

 746) has a tail with a notochord (noto.), and a body in which the 

 characteristic muscular bands soon make their appearance. This 

 tailed larva becomes the asexual stage or " nurse." By and by 



