ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 587 



pharynx begins to function, tlie migratory cells which are scattered in 

 the syncytial mass continue to divide and increase considerably in 

 number. 



The straight or rhabdocoelic intestine becomes dendrocoelic by the 

 development of septa, which arise from the periphery and make 

 their way towards the interior in a manner which recalls the septa of 

 Anthozoa. The permanent endoderm is formed of the migratory cells 

 which lie on the internal surface of the walls of the body, and not, as 

 Metschnikoff thinks, of yolk-cells swallowed by the embryo. The 

 rhabdites, the brain, and the organs of sense are developed at the expense 

 of the cells of the connective reticulum. 



M. Hallez recognizes, in fresh-water Planarians, only two layers — the 

 ecto- and endoderm. The migratory cells which give rise to various 

 organs are regarded as homologous with the " pseudomesoderm " of 

 Coelenterata, the nutrient syncytial mass corresponding to their 

 gelatinous mass. With regard to the affinities of the Turbellaria, 

 the author admits that the solid mesoderm of the Pseudocoelia 

 of the Hertwigs is homologous with the mesoderm of the Entero- 

 coelia ; that the pseudomesoderm of Ccslenterates and the cutis- 

 cells of Echinoderms are homologous ectodermic differentiations, and 

 that the gastric diverticula of the Ctenophora are homologous with those 

 of Echinoderms. Starting with these bases he divides multicellular 

 animals into four groups : — (1) Mesozoa, characterized by ectoderm and 

 endoderm only ; (2) Porifera or Coelenterata, characterized by ectoderm, 

 pseudomesoderm, and endoderm; (3) Ctenophora and Echinodermata, 

 with the three germinal layers and pseudomesoderm ; and (4) the rest of 

 the Metazoa, with three layers but no pseudomesoderm. Having shown 

 that most of the Polyclades have a true mesoderm, while the Triclades 

 and Stylochus have not, he concludes that the Dendroooela which possess 

 a primitive mesoderm, ought to be associated with the fourth group, 

 while the Dendrocoela which have only a pseudomesoderm, ought to be 

 associated with the second, and more particularly with the true Coelen- 

 terata. 



From this point of view the connective reticulum of the Polyclades 

 is seen not to correspond morphologically to that of the Triclades ; this 

 is supported by a number of anatomical and embryological facts. M. 

 Hallez is not convinced by the arguments which have been adduced in 

 favour of the descent of the Dendrocoela from the Ctenophora, but thinks 

 that the ancestors of the former must rather be sought for among the 

 Anthozoa. 



The Ehabdocoela, as much as the Dendrocoela, may be divided into 

 two groups, according as they do or do not possess a pseudomesoderm. 

 The Microstomea will probably be found to be allied to Hydra or 

 Protohydra. 



Lateral Organs.* — Prof. W. Salensky discusses the homology of the 

 lateral organs of Nemerteans in connection with an observation made by 

 the brothers Sarasin on peculiar " cerebral tubes " in the embryos of 

 Helix waltonii. These tubes arise on each side of the cerebral mass as 

 two invaginations of the sensory plates, and were compared by the dis- 

 coverers with the smelling organs of some Annelids (e. g. Lopadorliyn- 

 chus). With this Salensky entirely agrees. He goes further, however, 



* Biol. Centralbl., viii. (1888) pp. 79-80. 



