538 



COMPARATIVE ANATOMY 



CHAP. 



The rudiment of the primary stone canal appears at the blind end of the left 

 limb of the hydrocccl, as a dorsal process, running to the left. 



The tubular parietal sinus, which is now completely isolated from the hydroccel, 



Fig. 4-lT.— Median, longitudinal section 

 tlirougli a free- swimming larva of An- 

 tedon, twenty - eight hours old, in the 



act of becoiniity; attached (after Seeliger). 

 [-V, The ciliated rings ; 1, neural plate with 

 nerve fibres (2), and ganglion cpIIs (3) ; 



4, gelatinous nucleus, the mesenchyme 

 cells which crowd it are not represented ; 



5, the tubes of the chambered organ ; 6, in- 

 testine ; 7, right coelom ; 8, vestibule ; 

 9, parietal sinus ; 10, right enteroccel ; 

 11, hydroccel ; 12, adhesive pit ; 13, left 

 enteroccel. 



Fig. 448.— Free-swimming larva of 

 Antedon, forty - eight hours after 

 heing hatched, from the left side, 

 specially to illustrate the rise of the 

 skeletal plates. I-V, The ciliated rings ; 

 6ai-&a5, the five basals ; ori-or^, thp five 

 orals, those lying on the right side re- 

 presented as discs ; 1, vestibule ; 2, in- 

 testinal vesicle ; 3, right enteroccel ; 



4, calcareous joints of the stalk ; 



5, pedal plate. 



has shifted to a position in front of and above the latter. Its posterior end grows 

 out till it touches the ectoderm immediately in front of the fourth ciliated ring 

 ventrally and to the left, and finally breaks out through the hydropore at this point. 



3. Attachment of the Larva and its Transformation into the Stalked Form 



(Figs. 449-453). 



Attachment takes place by means of the adhesive pit, which yields a sticky 

 secretion ; and since this pit lies ventrally at the anterior part of the body, the 

 attached larva has at first a position parallel to the surface to which it is fastened, 

 and the vestibule lies immediately above that surface. The body, however, soon 

 becomes erect, and the adhesive pit takes up a terminal position. 



Very soon after attachment the ciliated rings disappear, and so does the 



