A Contribut. to the Eutoiuol., Lifc-history, and Classificat. of the Dicyetnids. "]] 



recognìzing the fact establislied by the Hertwig's (31) , that the forina- 

 tions which arise between the ectoderm aud endoderm do iiot bave the 

 sanie morphological vaine by ali Metazoa, Van Beneden stili maintains 

 that the Dicyemids are clearly distinguished from each of the tliree 

 metazoic divisions, Coelenterata, Pseudocoelia, and Enterocoelia^ sinee 

 they present neither mesenchym nor coelomic lamellae, nor 

 even-ft strnctureless layer of any descri}»tiou between the ectoderm and 

 endoderm. 



As to the possibility of degeneration by parasìtism, weare told that 

 »dans le développement d'nn Dicyémide, rieu n'indique nne déi;énéres- 

 cence et le fait du parasitisme ne suffit pas pour affirmer une rétro- 

 gradation«. (p. 219.) 



Julin's interesting researches on the Orthonectidae (21) bave led 

 him to the conclusion that these parasites are very closely allied to the 

 Dicyemids, as shown both by their developmental history and their adult 

 structure. As to the necessity, or propriety, of establishing a middle, or 

 mesozoic division ot the animai kingdom, Julin adopts, without reserve, 

 the opinion of Van Beneden , claiming that the Oi thonectidae as well 

 as the Dicyemids are diploblastic animals , and as such must be sepa- 

 rated from the Metazoa. 



Van Beneden (2) has described two rare and peculiar forms , Co- 

 nocyema polymorplia from Octopus vulgaris^ and Microcyema vespa from 

 Sepia officinalis^ to which he has given the family uame Heterocyemidae . 

 The Dicyemidae and the Heterocyemidae unite to form the order, Rhomh- 

 ozoa ; and the Bhombozoa with the collateral order , Ortìionectida con- 

 sti tute the new type, Mesozoa. 



Such is the imposing edifice of names placed on the backs of two 

 minute parasites whose life-history is stili, in some of its chief features, 

 a complete enigma. Although our knowledge of the Dicyemids and the 

 Orthonectidae is not suffìciently complete to enable us to point out with 

 certainty their proximate allies , it is nevertheless ampie enough for 

 forming a tolerably clear uotion of their more remote aftìnities. Van Be- 

 neden contends that these animals cannot be included among the Me- 

 tazoa , without modifying the definition of the latter. One might ask, 

 what possible objection can there be to a modifìcation of this definition ? 

 So far as the mesoderm is concerned , the necessity for a modifìcation 

 has already arisen ; and I see no reason why the definition of Metazoa 

 should be less elastic than that of Mesozoa, which has just been remod- 

 elcd for the reception of the Orfhomef'idae. Rvery question of this kind 

 has at least two sides, the theoretical and the practical. The idea of a 

 mesozoic type is, in itself, unobjectionable. No one will deny that there 



