264: INVERTEBRATE MORPHOLOGY. 



corona {cor). The cavity of the dome is occupied by the U- 

 shaped digestive tract (s), the mouth and anus both opening 

 within the circle formed by the corona, a deep depression, 

 the vestibule, lying between the two. In the coelom above 

 the floor of the vestibular depression are a number of meso- 

 derm-cells, and also a ciliated canal composed of a single row 

 of perforated cells and probably excretory in function. Upon 

 one surface of the larva between the marginal ciliated band 

 and the apical thickening is a peculiar glandular organ termed 

 the cement-gland {pyr), around the mouth of which are situ- 

 ated a number of strong cilia. 



The development of this larva into the adult form is accompanied by a 

 number of remarkable changes, which in their details and significance are 

 not yet thoroughly understood. The larva settles upon the ventral or oral 

 face and shortly thereafter one wall of the vestibule becomes pushed over 

 towards the other and eventually unites with it, the original vestibular 

 cavity becoming divided into two portions, one of which remains in con- 

 nection with the surface of fixation and later degenerates, while the other 

 has opening into it the mouth and anus, though the former opening at 

 about this period becomes closed. Later a remarkable rotation through 

 180° of the digestive tract, together with the portion of the vestibule in 

 connection with it, occurs, the portion of the body immediately above the 

 point of fixation elongating to form the stalk of the adult, becoming at the 

 same time filled with mesodermal tissue. The mouth opens again into the 

 vestibular cavity, the tentacles arise from the wall of the cavity which 

 later opens to the exterior, the adult form being thus assumed. The fate 

 of the apical thickening and of the cement-gland is uncertain ; they have 

 been described as degenerating without taking any part in the formation of 

 the adult organs, though it has been suggested that the apical thickening 

 may give rise to the nervous system of the adult. 



The form of the larvae in the Bctoprocta is subject to con- 

 siderable modification. In Menibranipora and some other 

 genera the larva is known as the Cyphonautes (Fig. 117 B) 

 having been so designated before its life-historj' was eluci- 

 dated. It has a somewhat triangular outline and is character- 

 ized by being enclosed in a bivalved chitinous shell (sh). At 

 the apex of the triangle is the apical thickening {cal), with its 

 elongated cilia, while around the base there is to be found the 

 corona (cor). A well-developed digestive tract is present, both 

 the mouth and anus opening upon the basal surface of the tri 

 angle, and therefore within the area enclosed by the corona 



