A. E. Yerrill — The Ojpisthoteuthidce. 



79 



before tbe foot appears. The latter first arises as a small ven- 

 tral protrusion, between the mouth and anus, which are very 

 near together, in the subveliger stage of nearly all gastropods 

 and bivalves. 



/\-*i' 



Figure 6. — Diagramatic figures, to show axes. I, gastropod ; II, cephalopod ; 

 A, P, longitudinal or antero-posterior axis, D,Y, dorso-ventral axis, according 

 to Lancaster; L,L, longitudinal axis as understood by the writer; a, mouth; e, 

 anus; g, gill cavity; /, foot; s\ siphon. (Copied from Lancaster with addition 

 of lines L,L.) 



Taking this as a true exposition of the relation of parts in a 

 gastropod, it is easy to prove that the body of a cephalopod 

 corresponds to that of a gastropod, and that the actual dorsal 

 is the true dorsal side ; that the pedal organs (arms, web, 

 siphon) are anterior and antero-ventral in all ordinary adult 

 forms ; * and that the intestine makes a sharp turn and termi- 

 nates in the median line, below the neck, at the base of the 

 siphon, f and, therefore, behind the antero-ventral foot, 

 which, indeed, is the primitive position in the larval state of 

 most mollusca, and may, therefore, be called the normal posi- 

 tion. 



In Opisthoteuthidce, on the contrary, the siphon and bran- 

 chial cavity have been pushed backward to the posterior end 

 of the body, far away from the mouth, owing to the peculiar 

 development of the arms and web and their extensive union 

 with the ventral surface of the body. The intestine has, 

 therefore, lost its usual forward bend. Thus the family, in 

 these respects, stands in about the same relation to other cephal- 

 opods that chitons do to ordinary gastropods. 



These peculiarities seem quite sufficient to warrant the 

 forming of a new family group. The Opisthoteuthidse agree 

 with the Cirroteuthidse in having a single row of suckers on 

 each arm, with two lateral rows of slender cirri ; in having a 

 broad web ; in the presence of lateral fins ; and, apparently, in 

 lacking an odontophore. 



The exceptional positions of the anus, siphon, gills, sexual 

 and renal organs, in this family, make it agree, in these respect, 

 with Lancaster's hypothetical " arche-mollusc.":j: However, in 



* In the embryo (squids, etc..) the arms arise below and behind the mouth and 

 only surround the oral disk at a later period. 



f The two folds that arise on each side of the embryo and finally unite to form 

 the siphon are regarded as lateral parts of the foot. 



X Eneyclop. Brit. vol. xvi, p. 615. It should be stated that Lancaster, himself, 

 does not claim that his " schematic mollusc" or " archetype " is such in the sense 

 of a real primordial ancestral mollusk. 



