62 



LOWER INVERTEBRATES. 



Order II. — OLYNTHOIDEA. 



These forms differ from those of the last order, in having a skeleton of calcareous 

 spicules. These may be either straight and needle-like, or one end may bear three or 

 four rays. The spicules, which are of mesodermal origin, are arranged at right angles 

 to the inner and outer walls of the body in the tube or vase-shaped forms, and the 

 rays being interlaced, afford a firm scaffolding for the support of 

 the walls. Here, as among the Physemaria, Haeckel claims that an 

 outer epithelial membrane is absent, but Grane, Schultze, Metschni- 

 koff, and others have repeatedly demonstrated the existence of a 

 true ectoderm, and the writer has seen this membrane several times 

 in the living sponges. The species are usually colorless, generally 

 of small size, and although abundant along our coasts must be 

 looked for carefully under stones and upon sea^weeds. They are 

 found exclusi-sely in shallow water, and, with few exceptions, do 



Fig. 54. — Ascaltis. one ^ n t i j.^ 



of the Asoones group, not occur on muddy bottoms. 



Sub-Order I. — Ascones. 



These forms have a vase-like shape, are thin-walled, and have a distinct skeleton 

 formed of a single layer of triradiate spicules, their bases outward, while the pores of 

 the supply system are formed as they are needed, through 

 the sides. The inner or ca'lomatic cavity is lined with 

 flagellated and collared cells, while these, of course, 

 are not found in the transient supply-canals, which, 

 according to Haeckel, are but temporary 02oenings in 

 the sides of the sponge. 



Sub-Order II. — Sycones. 



The typical form for the members of this group is 

 that shqwn in the figure of Scycandra ciliata. The in- 

 dividuals are attached by the base or 

 small end, and are very like those of the 



Ascones, but are stouter and more frequently spindle-shaped, while the 

 walls are thicker and more opaque. They are, however, quite dis- 

 tinct in their structure. The flagellated and collared cells are con- 

 fined to the cavities of the permanent supply canals, where they occujjy 

 special cavities, the ampullaceous sacs. The cells of the coelomatic 

 cavity are flattened and similar to those of the ectoderm. The meso- 

 derm is very thick, and the canals radiate with gr-eat regularity from 

 the cavity of the sponge to the exterior. The outer part of each canal 

 represents the supply, and the inner, the drainage system. The spic- 

 ules are usually in two rows, their radiated bases being turned, respec- 

 tively, inwards and outwards. While the living species are some- 

 what numerous, but one fossil genus is known, and this is of Jurassic age. 



-Diagrammatic section of 

 Ascaltis. 



Fia. 56. —Scycan- 

 dra ciliata. 



