32 ZOOLOGY 



SECT. 



enclosed in a layer of cells. Parallel with this runs, on the dorsal 

 side, the narrow caudal portion of the nerve-cord, and at the sides 

 are bands of muscular-fibres. In the trunk the nerve-cord is dilated 

 to form the ganglion of the trunk, and, further forwards, expands 

 into the sense-vesicle (sens, ves.) with the otocyst (oto.) and eye 

 (eye). The enteric canal is distinguishable into pharynx, oeso- 

 phagus, stomach and intestine. The pharynx opens on the exterior 

 by the mouth : in its ventral floor the endostyle (end.) has become 

 developed ; its walls are pierced by stigmata, the number of which 

 varies ; a ciliated sac (cil. gr.) opens into it below the trunk part of 

 the nerve-cord. 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 become developed. In this tailed 

 free-swimming stage the larva remains only a few hours ; it soon 

 becomes fixed by the adhesive papillaB, and begins to undergo the 

 retrogressive metamorphosis by which it attains the adult condition. 



The chief changes involved in the retrogressive metamorphosis 

 (Fig. 691 ) 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 trunk part of the nervous system to a single ganglion, and 

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

 free-swimming larva, with well-developed 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 be- 

 come aborted. The significance of these facts will be pointed out 

 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 one genus of the simple 

 forms (Molgula), in which the tailed stage is wanting. 



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 forma- 

 tion of constrictions, into four parts, each destined to give rise to 

 a zooid ; and this group of tetrazooids, as they are termed, gives 

 rise by budding to an entire colony. 



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. 

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

 characteristic muscular bands soon make their appearance. By 



