PHYLUM CHORDATA 



39 



In Pyrosoiua 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 segment- 

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

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

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

 rise to a zooid (tctrazooid). 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 Bolwluin 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 



Fin. r37.-Doliolum, late stage in the devlopment o( the tailed larva, ah: ap. atrial arerture ; 

 <;<„■«..<?. uadnphnre; end. cndostyle ; M. heart; ,ie. fl"- ncrve-galighon ; nolo, notochord, 

 or. ap. oral aperture ; rent. .it. ventral stolon. (After Uljanin.) 



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

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

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

 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 surfkce of the parent (Fig. 738, c, and Iig 739) till 

 they reach the cadophore, to which they attach themselves alter 

 multiplying by division. The cadophore soons becomes elongated, 

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

 the lono- chain of zooids thus established is further developed, 



