434 NOTES AND MEMORANDA. 



takes place, and the egg escapes, to be seized on by tbo " claspcrs." 

 The plan of development of the sperm-cells is of the same character ; 

 after impregnation the endoplasm of the egg is seen to become 

 possessed of a number of cells which move towards the periphery and 

 pass into the ectoplasm, where they form the blastoderm ; between 

 the cells of these two regions a differentiated lamella soon appears, 

 which will go to form the future supporting lamella : the endodermal 

 mass becomes fissured in anticipation of the gastric cavity, while the 

 ectoderm gives rise to ingrowths, which are soon converted into the 

 larval tentacles of Mijriothda. The free actinula-enibryo has no mouth, 

 and nutrition is effected by the remnants of yolk-sj)heres in the gastric 

 cavity. As soon as the mouth is developed, the embryo becomes fixed, 

 the primitive tentacles are absorbed, and the blastostyles developed. 

 Compared Avitli the calcareous sponges, Myriothela differs in the 

 characters of its gastrula ; but as is pointed out, these animals have a 

 simple ovum without any nutrient material, while the siliceous sj)ouges 

 which have an ovum provided with nutrient material, do also form a 

 " planula." 



The mesoderm is stated to be incompletely differentiated, and to 

 consist of two muscular layers, the upper of which is distinctly sepa- 

 rated from the ectoderm, while the lower is intimately connected with 

 the endoderm ; these two layers are separated by the supporting 

 lamella which is likewise mesodermal in origin. 



Porifera. 



Structure of the Aplysinidae. — This family of horny sponges is 

 treated of by Professor Eilhard Schulze in one of his important series 

 of " Researches on the Structure and Development of Sponges." * The 

 first part of the paper consists of an historical resume of former work 

 on the family, and enumerates the genera belonging to it, namely, 

 Aphjsina, Verongia, Dendrospongia, Danoinella, Janthella, and the 

 author's new genus Aplysilla. To this follows an anatomical and 

 histological description of Aphjsina and of Apli/silla. 



1. Aphjsina aerophoba. — Of the account of this already well-known 

 species, our space will only allow of our noticing tlie histological 

 portion. 



The ectoderm consists of a single layer of flat jiolygonal cells with 

 distinct nucleus and nucleolus. The boundaries between these cells 

 are often seen only on treatment with silver nitrate, and in some 

 cases even this treatment failed to reveal cell contour, owing to the 

 presence of a structureless cuticula, probably formed under abnormal 

 conditions for i^rotective purposes. 



The mesoderm consists of a sort of connective tissue made up of 

 cells imbedded in a matrix, which is jiartly granular, partly (in the 

 neighbourhood of the external surface and of the canals) hyaline. 

 The cells themselves are mostly stellate, being jjrovided with long, 

 sometimes branching processes. But where the mesoderm surrounds 

 the water canals, these cells become spindle-shaped, and surround 



* ' Zeitsch. f. wiss. Zcol.,' xxx. (1878) 379. 



