COMPARISON WITH ENTEROPNEUSTA. 



149 



anterior coeloms unite in the front part of the preoral lobe to form a 

 single cavity.* 



In bipinnariae with brachiolar arms, which are it will be remembered 

 processes of the preoral lobe, temporary fixation f during development 

 takes place by these arms. During the metamorphosis of bipinnariae 

 fixation occurs by means of a median 

 oval sucker-like disc on the preoral 

 lobe (p. 141). Attachment by the 

 preoral lobe has also been noticed by 

 Perrier in Asterias spirabilis, in which the 

 larvae adhere to the buccal membrane of 

 the mother. 



From the above account f it is 

 clear that in the larvae of Aste- 

 roids which become attached dur- 

 ing development, the fixation is 

 effected by the preoral lobe, and 

 that the stalk so formed is sur- 

 rounded by the hydrocoel and 

 springs from what will become the 

 oral surface of the starfish. The 

 knowledge of this fact, so impor- 

 tant for a proper comprehension of 

 the morphology of the class, we 

 owe to MacBride. The further 

 statement may be made that the 

 coelom, which arises by a single 

 diverticulum from the enteron, 

 becomes segmented into three 

 pairs of chambers, viz;, the anterior coelom, the hydrocoel, and 

 the posterior coelom. Of these the anterior coelom is at first 

 single or soon becomes so, and, as in the Enteropneusta, acquires 

 an opening or two openings to the exterior (water pores). Of the 

 second pair the chamber of the left side becomes much larger 

 than that of the right, retains its connexion with the anterior 

 chamber by the stone-canal, and becomes the hydrocoel, while 

 the right remains as a small apparently functionless sac ; this 

 pair of chambers may be compared to the collar-cavities of the 



* There are according to Field (op. cit. ) two water-pores in many of the 

 bipinnariae of Asterias vulgaris, but it is doubtful if this can be regarded 

 as a normal occurrence (see note, p. 166). 



t Bury, op. cit., 1895 ; Delage, Arch. Zool. Exp. (4), (2), 1905, p. 27. 



f The preceding account of the development of Asterina is taken largely 

 (often directly quoted) from the important work of MacBride (loc. cit.). 



4-f \- 



FIG. 106. Optical section of an Asteroid 

 larva (Bipinnaria) seen from the dorsal 

 surface (after Bury). There are two 

 anterior coelomic sacs, the water 

 pore is shown opening into the left of 

 them. The hydrocoel is indicated but 

 not yet completely separated from the 

 left anterior coelom. 1 left anterior 

 coelom ; 2 water pore ; 3 hydrocoel : 



4 second terminal plate (arm B) ; 



5 left posterior coelom ; 6 mesentery 

 separating the two posterior coeloms ; 

 7 right posterior coelom ; 8 right 

 anterior coelom ; 9 euteron. 



