640 



INVERTEBEATA 



CHAP. 



learnt, the rudimentary embryo is called the Cyathozooid ; the stolon 

 gives rise to four " blastozooids " known as " Ascidiozooids," which 

 form a cycle according to Juhn (1912). The ectodermal peribranchial 

 or atrial tubes of the Cyathozooid extend backwards into the 

 Ascidiozooids, and give rise to the atrial cavities of these persons by 

 segmenting off four pieces; but this, according to Hjort, does not hold 

 ibr buds subsequently produced. The atrial cavities of the later buds 

 arise from solid proliferations of the walls of the inner vesicle, which 

 subsequently become hollowed out, and obviously correspond to the 

 lateral compartments of the inner vesicle of the bud of Distaplia. 

 The nervous system arises as a pair of outgrowths from these two 

 lateral vesicles, which grow inward and unite in a mid-dorsal tube, 

 which then forms a new connection with the central vesicle in front. 



Fig. 465. — Lateral view of old bad of Distaplia magnilarva. (Alter Hjort.) 



at, opening of single atrial cavity ; andnt, endostyle ; gang.ad, adult ganglion, a thickening of the 

 dorsal wall of the hypophysial tube ; g.s, first gill-slits appearing as perforations of the pharyngeal wall ; 

 hyp, hypophysial tube ; m, mouth. 



The posterior connections become solid and form a transverse 

 nerve (Fig. 466). 



In Salpa, the buds formed by the stolon remain connected with 

 one another for a considerable time, and constitute in this way a 

 train of "blastozooids," which is dragged along by the oozooid, as 

 the person produced from the egg is termed, and which thus acts as 

 engine to the train. As the blastozooids attain a respectable size 

 they drop off and lead independent lives; they constitute the sexual 

 ' generation. 



In Doliolum the buds break loose from the stolon whilst they are 

 in a most immature condition, and climb by amoeboid movements 

 over the surface of the parent, till they reach a posterior horn of the 

 test situated in the mid-dorsal hne. To this horn they attach them- 

 selves in three longitudinal hues. Those in the lateral lines become 

 trophozooids, without muscles and with wide mouths, which serve 

 to nourish the colony; those in the median Hne become like the 



