A contribut. to the Embryol., Life-history, and Classitìcat. of the Dicyemids 9 



pushed forward until , in extreme cases , ali the polar cells come to He 

 in the same oblique piane, which looks toward the ventral side. 



The genus Dicyemennea embraces those specie» in which the ca- 

 lotte is composed of nine cells, four propolar and fìve metapolar. D. 

 Millìeri , one of the thvee species included in this genus , is somewhat 

 doubtfully placed , as the number of cells in the calotte has not been 

 ascertained. The form of the calotte, as given in the figures of Clapa- 

 EÈDE and Laohmann, makes it quite certain that it does not belong to 

 the octamerous type; while its dose resemblance to the enneamerous 

 calotte of D. Eledones leaves but little doubt in regard to its generic 

 Position. 



The differeuce of a single polar cell might; at first, appear insuffi- 

 cient for a generic mark , especially when estimated by any standard 

 that would apply to one of the higher animals. Any objection of this 

 kind will probably disappear on reflecting that every celi in a pluri- 

 cellular animai has, in addition to its morphological and physiological 

 value, a numeri cai and topical value. The numerical vaine Stands in 

 inverse ratio to the total number of cells composing the organism, and 

 where the latter never exceeds 25 — 30 cells, the former must be vcry 

 considerable. The topical value will also be greater in proportion as 

 the total number of cells is smaller and the dififerentiated regions fewer; 

 and , further , greater in proportion to the morphological and physiolog- 

 ical importance of the region in which it is placed. The calotte of a 

 Dicyema is the most highly dififerentiated part of the animai, and hence 

 a difference of a single celi in its coraposition must be admitted to be of 

 more importance to the systematist than tiie sanie numerical difierence 

 in any other region. A comparison of the octamerous with the ennea- 

 merous calotte will show that we bave not over-estimated the funda- 

 mental difference between them. The added celi gives a calotte dis- 

 tinctly characterized in form and structure , and further distinguished 

 by being permanently orthotropal. 



Systematic Descriptions. 

 1. Dieyema typus E. v. Ben. Figs. 16 — 21, PI. 2. 



Found only in Octopus vulgaris Lam. Calotte octa- 

 merous; orthotropal in the younger, and many of the 

 medium-sized indivi du als, but strougly plagiotropal 

 in many of the adults measuring about '2.5mm in leugth; 



