C. Porifera calcaria. \ ] 



IVIinchin (') publishes some histological details of Leucosolenia clathrus. The 

 ec to derma is a contractile layer, which in the expanded state of the sponge 

 consists of flattened, nonciliated cells. When these cells contract, their nuclei 

 remain in the proximity of a limiting membrane which is seen immediately under 

 the ectoderm . The protoplasm thickens externally from the nucleus and at the 

 periphery it expands like a membrane. The cell then has assumed the mushroom 

 shape. The author never saw any structure resembling a duct, as described by 

 Bidder. Pores in every stage of formation are visible at the ends of grow- 

 ing branches. The first stage is an ectoderm cell which is somewhat more 

 granular than the ordinary cells, with protoplasm of a characteristic yellowish brown 

 tinge, and having distinct cell limits. Such a cell then grows inwards, and reach- 

 ing the endoderm pushes its way between the choauocytes, while still connected 

 with the pinacocytes. After this stage the cell spreads out and becomes perforated. 

 The fully formed pore is a single cell with a nucleus and an intracellular duct, 

 which has a wide inner opening and a very delicate outer opening. - - The spi- 

 cules always have on each arm one or two cells, the protoplasm of which is with- 

 out granules. Besides these cells the parenchyme contains ova and potato - 

 shaped wandering cells of a greenish colour, full of refractive granules and with 

 the nucleus hardly visible. The stellate mesoderm cells . . . are exceedingly 

 rare, if not entirely absentee. - The choanocytes vary in shape, according as 

 the sponge is expanded or contracted. In the first case they are short and broad, 

 with long collars; in the contracted state they are narrow, compressed, with low 

 collars. The nucleus lies at the base of the cell; above it is a bright space which 

 is perhaps a kind of food vacuole. The flagellum is of equal thickness throughout. 

 The collar is thickened towards the base and exceedingly thin towards the ex- 

 tremity. 



Minchin ( 3 ) found in Leucosolenia coriacea (Mont.) Bwk. a sieve-like membrane 

 stretched across the cloacal tube near to the osculum and just above where the 

 choanocytes end. This sieve membrane is composed of two layers of cells, 

 separated by a thin layer of jelly. The cells have a central portion containing 

 a nucleus and possessing 4 to 6 processes , which unite with others , thus forming 

 a network with rather wide meshes. The author believes that the membrane is 

 formed by the gastral cavity breaking through to the exterior in several places 

 during development and that its inner layer of cells is endoderm, the outer layer 

 ectoderm. The membrane is compared to that of Olynthus-like stages of Sycandra, 

 on the other hand with the well-known membranes of Euplectella , Holascus and 

 Hyalonema, without stating, however, that these formations show true homologies. 

 The author could not find the membrane in L. primordialis nor in botryoides. - 

 Histological details of L. coriacea: NoSollas's membrane present. The flagellum 

 of the choanocyte has usually twice the height of the cell or more ; it is of pre- 

 cisely the same thickness throughout*. No continuation of it into the interior of 

 the cell was seen. In macerations the author found the spicule to have a nucleus 

 at the extremity of each ray and a fourth at the confluence of the ray set. Some 

 of the ectoderm ces are perforated, forming a pore [see , supra plO Bidder (^J. 



IVIinchin ( 4 ) describes a ring-like ridge into the interior of the osculum of Leuco- 

 solenia clathrus. It is composed of two layers of ^ectoderm , with a few scattered 

 amoeboid cells between. These ectodermal cells are the contractile elements. 

 This sphincter is physiologically similar, but morphologically quite different^ 

 from the sieve-membrane in L. coriacea [see supra]. By means of the sphincter 

 the osculum can be narrowed and even quite shut. But the tubes can also con- 

 tract very greatly; the result is that the external appearance of the sponge is quite 

 modified. In highly contracted specimens the mesoderm is thickened and the 



