A Contribut. to the Embiyol., Life-histoiy, and Classificat. of the Dicycmids. 29 



The points of agreement are : 



1 . Total number aud arrangement of cells. 



2. Form and general aspect. 



3. Number and disposition of tlie polar cells. 



They dififer, as a rule, in the following particulars : In D. gracile 



1. the head is smaller and more spherical. 



2. The fore ends of the metapolars are thicker than the hind ends. 



3. The parapolars are not thickened at theìr junction with the 

 calotte, and consequently take no part in forming the head. 



The more common form of the head is seen in fig. 49, and an ex- 

 ceptional form in fig. 48. 



The following forms (1—5) were ali sketched from large ìndivid- 

 uals found in the same Sepia. 



Such forms as 1, 2, and 5 bave never been seen in D. Eledones. 



Since this chapter was written, a second paper by Van Beneden 

 has appeared (No. 2), in which he describes two new genera, Cono- 

 cyema and Microcyema. As these forms are supposed to represent a 

 family distinct from that of the «true Dicyemids« , it does not appear to 

 be necessary to make any changes in the foregoing pages. 



Chapter II. 



Reproductiou. 



The two kinds of embryos produced by the Dicyemids were seen 

 and described by Erdl nearly forty years ago, but were supposed by 

 him to represent different stages in the development of one and the 

 same individuai. 



KÖLLIKER was the fir^t to show that these embryos are two entirely 

 distinct forms , arising from two different germs , which pursue two 

 unlike courses of development while within the parent body. He ascer- 

 taiued further that the twu sorts of embryos, which he distinguished as 



