MESOZOA. 815 



develope within an axial cell. The male has the form of a top ; certain non-ciliated 

 cells at its anterior extremity inclose refractile bodies ; females viviparous.' There 

 are two families, Dicyemidae and Heterocyemidae. 



Van Beneden considers that there are five genera of Dicyemidae^ each genus 

 confined to a genus of Cuttlefish, and a given species of each genus confined in 

 turn to a given species of the corresponding genus of Cuttlefish l . Whitman has 

 however proved (i) that the same Cuttlefish may harbour two Dicyemids belonging 

 to different genera ; (2) that the same species of Dicyemid may be found in different 

 species of the same genus of Cuttlefish, or even in different but closely allied genera 

 (Sepia and Rossia) ; (3) that two closely allied species of Dicyemids occur in 

 Cuttlefish belonging to different families, while two remotely allied species occur in 

 two closely related genera of Cuttlefish (Sepia and Rossia). Whitman consequently 

 proposes to range the known species of Dicyemids under two genera, Dicyema and 

 Dicyemennea, characterised solely by anatomical differences. The largest species 

 described by Whitman (Dicyema macrocephalum) attains a length of 5-7 mm. : the 

 smallest (D. truncatuin) only -5075 mm. 



Every Dicyemid consists of a head or calotte, and a more or less elongated 

 body. The calotte is composed of two circlets of ectoderm cells, an anterior circlet 

 of four propolar cells, a posterior of four metapolar cells in Dicyema, or of five, three 

 dorsal and two ventral, in Dicyemennea. In the first-named the calotte is always 

 symmetrical in the young animal ; its cells arranged round an axis continuous with 

 that of the body, hence orthotropal ; but in the adult it becomes oblique and bent 

 towards the ventral aspect, hence plagiotropal. The calotte is followed by two 

 parapolar ectoderm cells, always lateral in position, one on the right, the other on 

 the left. The remaining ectoderm cells cover the body : their number varies in the 

 different species. The caudal extremity is formed by two cells, of which one is 

 usually dorsal, the other ventral. The ectoderm cells are ciliated on their external 

 faces, are uninucleate, and contain granules of different kinds. Certain of them, 

 always few in number, often have their centres or posterior extremities swollen into 

 a wart, hence verruciform cells ; and in these warts are collected numbers of re- 

 fractile globules. The edges of the cells fit one with another. The axis of the 

 body is occupied by a single huge endoderm cell which extends from the propolar 

 cells to the caudal extremity, covered at every point by the ectoderm. It is in 

 shape a cylinder pointed at each end. Its protoplasm is largely vacuolated ; the 

 vacuoles contain a hyaline liquid, immiscible with water. Its nucleus is large, with 

 a tough membrane, and contains a nuclear network. 



Every Dicyemid, according to Whitman, is either monogenic or diphygenic. 

 The first-named produces only vermiform embryoes, and is hence a primary Nema- 

 togen. The second produces (i) infusoriform embryoes (or males, E. v. B.), and 

 then (2) vermiform embryoes ; it is consequently first a Rhombogen (E. v. B.), and 

 next a secondary Nematogen 2 . On this view the Rhombogen and secondary 

 Nematogen are successive phases in the life-history of one and the same individual, 



1 Van Beneden's names are as follows : Dicyema, confined to the Cephalopod genus Octopus ; 

 Dicyemella to Eledone ; Dicyemina to Sepia ; and Dicyemopsis to Sepiola. See the historical section 

 in Whitman's paper, Mitth. Zool. Stat. Naples, iv. 1883, p. 3. 



2 Rhombogen = individual producing infusoriform embryoes, Nematogen = individual producing 

 vermiform embryoes, accordirg to E. van Beneden's nomenclature. 



