510 SUMMARY OF CURRENT RESEARCHES RELATING TO 



though not complete similarity in developmental history between 

 the Plana'ria and the Ctenophora. In both cases (1) the endoderm 

 arises as four large pale cells, and this layer gives rise to a 

 quadri-radiate enteron, which is permanent in the Ctenophora, but 

 modified in the Planaria. (2) The gastrula arises by epiboly, and the 

 blastopore and permanent mouth are coincident in position. (3) 

 Stinging cells are to be observed in both. (4) The embryo has in 

 both a predominantly radial (symmetrical) arrangement, but in both 

 this is later on more or less completely modified into a bilateral sym- 

 metry. On the other hand — (1) There does not seem to be in the 

 Planaria more than a feeble indication of an aboral sensory capsule 

 with otoliths, such as seen in the Ctenophora. (2) A complete in- 

 vestment of cilia is but rarely found in the Ctenophora, e. g. embryo 

 of Eucliaris. (3) Nothing comparable to the eight ctenophoral plates 

 of the Ctenophora can be detected in the Planaria. The relations of 

 the Ctenophora and the Planaria are hardly to be doubted. 



Turning to the Nemertinea, we find that in these the quadri- 

 radiate symmetrical cleavage is confined to the very earliest stages, 

 the endodermal cells are small, and there is no kind of communi- 

 cation between the enteron and the ccelom ; _ on the other hand, 

 stinging organs are to be found on the proboscis, the blastopore and 

 permanent mouth are coincident, and, in fine, the Planaria in some 

 cases present intermediate conditions between the Nemertinea and 

 the Ctenophora. 



The chief objects of the author's investigations have been Lepto- 

 plana tremellaris, L. alcinoi, Eurylepta cristata, and Thysanozoon 

 diesingi. 



Eyes of Planarians.* — Former investigations f having done little 

 more than elucidate the external characters of these organs, J. Carriere 

 has applied himself to determining their intimate structure, especially 

 that of the nervous elements. The method employed was preserva- 

 tion by Lang's method, viz. a liquid composed of chloride of mercury 

 5 parts, glacial acetic acid 5 parts, water 100 parts; after twenty 

 minutes or half an hour the specimen was transferred to alcohol of 

 70 per cent. ; sections were made and stained with picrocarmine. 



In Planaria polychroa there is an optic ganglion immediately in con- 

 tact with each eye, on its outer side, and consisting of an external layer 

 of nuclei resembling those of the cerebral ganglion, and about ■ 008 mm. 

 in length, enclosing a larger mass of fine fibres. Among these fibres 

 are some which are strongly refractive, and pass in straight lines 

 inwards, swelling out, and ending in rather broad knobs within the 

 pigmented hollow (" pigment-cup "), and which they probably fully 

 occupy in life. The pigment mass consists of small globules, varying 

 in size from ^^ to ^^ mm. in diameter. The eye of Dendrocoelum 

 lacteum is double ; the pigment-cup is single, but has two posterior 

 openings instead of one. The eye of Leptojplana tremellaris, described 

 by Keferstein in very different terms, appears, however, to agree 



* Arch. Mikr. Anat., xx. (1881) pp. 160-74 (1 pi.). 

 f See this Journal, i. (1881) p. 605. 



