xiir 



PHYLUM CHORDATA 



39 



In Pyrosoina development is direct, witliuut 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 segment- 

 ation is menjblastic. A process, develoi)e(l at an early stage, 

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

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

 rise to a zooid (tctrazooid). The jn'imary 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 

 fi-om 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 foui- primary tetrazooids, the entire adult colony 

 is produced. 



The development of Doliolniii 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. 

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

 characteristic muscular bands soon make their appearance. This 



Fig. T37.— Doliolum, late stage in the devlopment of the tailed larva, at,: ap. atrial aperture ; 

 dors. St. cadophure ; end. eudostyle ; hf. heart ; ne. fin. nerve-gauglion ; nolo, notochord ; 

 or. ap. oral aperture ; vent. st. ventral stolon. (After Uljanin.) 



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

 the tail aborts, and two processes, one jDostero-dorsal, the other 

 ventral, known respectively as the cadophore (dors, st.) and the ventral 

 stolon {vent, st.), grow out from the body of the larva. On the latter 

 are formed a number of slight projections or buds. These become 

 constricted off, and in the form oF little groups of cells, each con- 

 sisting of seven strings of cells with an ectodermal investment, 

 creep over the surface of the parent (Fig. 738, e, and Fig. 739) till 

 they reach the cadophore, to which they attach themselves after 

 multiplying by division. The cadophore soons becomes elongated, 

 and the bud-like bodies attached to it develop into zooids. As 

 the long chain of zooids thus established is further developed, 



