564 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



sist is differentiated into a cortical and a medullary substance 

 (ectosarc and endosarc). The sharpness of demarcation of 

 the ectosarc from the endosarc varies. In Actinophrys sol 

 the two pass, imperceptibly, one into the other ; in Actino- 

 sphcerium, the change from the ectosarc into the endosarc 

 takes place within a narrow zone, everywhere equidistant 

 from the centre. The line of separation between the endo- 

 sarc and the ectosarc is best defined in the Acanthocystidce, 

 Heterophryidne, etc., but it arises only from a differentiation 

 of the protoplasm, and not from the development of a defi- 

 nite membranous investment around the endosarc. The nu- 

 clei lie in the endosarc. When only one exists it is usually 

 eccentric, and, when there are many, they are scattered irreg- 

 ularly. The ectosarc contains contractile, and sometimes non- 

 contractile, vacuoles, which last may also be met with in the 

 endosarc. The pseudopodia are thin, filiform, and radiate 

 from the body ; sometimes their surface presents moving 

 granules. They rarely branch or anastomose. In many cases 

 they present an axial substance which may be traced as far as 

 the" endosarc. The silecious skeleton may consist of separate 

 spicula or form a continuous shell. 



The Heliozoa propagate by simple division with or with- 

 out previous encystation ; and the products of division may 

 or may not become encysted. They may either pass directly 

 into the adult state or become monadiform active larva?, pro- 

 vided with two flagella, a nucleus and contractile vesicle, 

 which in course of time develop into the parent form. 1 



A completely new light has been thrown upon the vexed 

 question of the supposed sexual method of reproduction of 



1 As this chapter was passing through the press, Hertwig's monograph "Zur 

 Histologic der Radiolarien " has come into my hands. The Radiolaria are de- 

 fined as Rhizopods with pointed, branched, usually anastomosing and granular 

 pseudopodia, which proceed from a protoplasmic body inclosing either nu- 

 merous small heterogeneous nuclei, or a single larger highly-differentiated ve- 

 sicular nucleus. The protaplasm of the body is further separated into a pe- 

 ripheral non-nucleated and a central nucleated portion, by a membranous capsule 

 with porous walls. The capsule is invested by a homogeneous gelatinous sub- 

 stance ; the extracapsular protoplasm usually contains numerous Vellow cells. 



Propagation is effected (probably always) by the breaking up of the body 

 into unicellular monadiform embryos provided with a single flaorellum. As a 

 result of these investigations, Hertwig admits that the Radiolaria and the 

 Heliozoa are closely allied, and even suggests that the name of Radiolaria 

 should apply to both groups, which would then form the subdivisions of Heliozoa 

 and Cytophora. The Ridiolaria (Cytophora) are distinguished into Oollozoa 

 (with numerous small nuclei) and Collida with a single highly-differentiated 

 nucleus. 



