TYrE OF THE VESICULARIAN LARVA. 



31 



4. Type of the Vesicularian Larva. 



In the larvae of this type an excessive lengthening of the coronal 

 cells is found (Fig. 13). The whole larva in this way becomes greatly 

 elongated; the aboral and oral areas 

 are exceedingly circumscribed, and 

 the ectodermal furrow (ec), as in the 

 previous types, is covered by the cells 

 of the corona. Another characteristic 

 of these larvae which was observed 

 by Barrois in Serialaria lendigera 

 (No. 9) and by Ostroumoff in Vesi- 

 cularia (No. 26) is the small size of 

 the retractile disc (?*s) and the un- 

 usual depth of the mantle- cavity (m) ; 

 posteriorly, i.e., opposite to the ecto- 

 dermal furrow, it is so deep as to 

 extend beyond the middle of the 

 body. Observers are not quite unani- 

 mous as to the sucker. According to 

 Barrois, only a functionless vestige 

 of this organ (s) is retained ; Ostrou- 

 moff, on the other hand, found a 

 well-developed though not very large 

 sucker. 



Fig. 13.— Larva of Serialaria lendigera 

 (after Barrois). rs, retractile disc ; 

 s, vestige of sucker ; ec, anterior ecto- 

 dermal furrow ; m, mantle-cavity. 



In the same way, authorities differ as to the metamorphosis of the sucker, 

 though in other respects metamorphosis here takes a similar course to that 

 described for Bugula. Ostroumoff maintains that in this type also meta- 

 morphosis commences with the evagination of the sucker, while Barrois holds 

 that fixation takes place by means of two lobes that grow out from the lower 

 end of the ectodermal furrow and which belong to the corona. For details of the 

 remarkable manner in which the corona is reversed, its long cells being rotated 

 and bent round, we refer the reader to Barrois (Nos. 8 and 9). The polypide 

 is here said to arise not by invagination, but through the separation from the 

 retractile disc of a cell-plate which becomes invaginated later (Ostroumoff). 



5. Type of the Cyclostomatous Larva. 



The metamorphosis of the marine Cyclostomatous Bryozoa has 

 been investigated chiefly by Metschnikoff (No. 21), Barrois (Nos. 

 6 and 9), and Ostroumoff (No 25). The larvae of this type 

 (Fig. 14, .4) are distinguished by the presence of a large sucker- 

 invagination (s), and by the vestigial condition of the retractile disc 

 (r), which is found as an inconspicuous cell-mass at the base of the 



