324 INVERTEBRATE MORPHOLOGY. 



nerves pass dorsally, but no special parietal ganglia occur. 

 Two pairs of buccal ganglia are also present. Otocjsts are 

 present imbedded in the foot in the neighborhood of the 

 pedal ganglia, but no other special organs of sense, unless 

 the bunches of tentacles be considered such, occur. 



No special respiratory organs are developed, the mantle 

 probably subserving the respiratory function. The heart, a 

 simple invagination of the wall of the pericardial cavity, lies 

 in the posterior region of the body, on the dorsal surface of 

 the intestine. It possesses no auricle, but receives the blood 

 through small slits in its walls. There are no special blood- 

 vessels, but the blood circulates through a series of sinuses 

 traversing the body in various directions. 



A pair of nephridia occurs in the posterior region of the 

 body, opening to the exterior by a pore on either side of the 

 anus, but a communication with the pericardial cavity is said 

 to be wanting. However this may be, the right nephridium 

 (rn) serves for the exit of the reproductive elements, though 

 the exact method by which these latter make their way into 

 the duct is unknown. Between each nephridial pore and the 

 anus there is a pore which seems to be the opening of a short 

 tube which communicates directly with the schizoccelic sinus 

 surrounding the terminal portion of the intestine and places 

 it in communication with the surrounding water, a peculiar 

 arrangement which recalls the dorsal pores of the oligo- 

 chsetous Annelids. The Scaphopods are bisexual, and the 

 reproductive organs, ovaries or testes, are single, consisting of 

 long completely closed sacs with lateral diverticula, lying 

 along the posterior wall of the visceral hump. As already 

 stated, the reproductive elements after the rupture of the wall 

 of the reproductive gland make their way to the exterior 

 through the right nephridium. 



Development and Affinities of the Scaphopoda. — The larva of Denta- 

 lium, though presenting considerable resemblance to the Trochophore, 

 diifers from it nevertheless in several points of detail. It possesses a dis- 

 tinct apical tuft of cilia and the prototroch is present, though represented 

 by three or more circles of cilia-bearing cells. The mantle-folds develop at 

 a relatively early stage as two lateral folds, quite separate along the ventral 

 line, the fusion characteristic of the adult only appearing later. It is this 

 early development of the mantle-lobes and the multiplication of the proto- 



