226 CRUSTACEA. 



Sacculina interna. The endoparasitic larva now wanders from 

 the point at which the attachment of the Cypris larva occurred, 

 further into the host, until it reaches the ventral side of the intestinal 

 canal where its final fixation takes place. At the same time it sends 

 out an exceedingly wide-spread network of rootlets which penetrate 

 throughout the whole body of the host, covering superficially all the 

 organs and only leaving the heart and the gills unaffected. At the 

 point where the actual Sacculina lies, all the rootlets gather together 

 to form a plate (basal membrane, Fig. 109, B\ in the middle of 

 which can be seen a swelling (central tumour ', Fig. 109, C). The 

 body rudiment of the Sacculina lies sunk in this central tumour 

 as its so-called nucleus. The rootlets, the basal membrane, and the 

 central tumour show essentially the same histological structure. They 

 consist of a superficial epithelium (ectoderm) and an inner cavernous 

 tissue composed of star-like anastomosing cells of connective tissue. 



The nucleus (Fig. 108) is completely sunk in the central tumour, and therefore 

 lies in a cavity which is called by DELAGE the perivisceral cavity (p), and which 

 opens externally through one small aperture. Even this aperture closes (Fig. 

 108, B], to re-appear later in the form of a transverse fissure (Fig. 109, /, fente 

 de sortie). The point at which the nucleus is in contact with the wall of the 

 perivisceral cavity is now known as the stalk (peduncle) of the nucleus. 



In the nucleus itself, a superficial ectodermal layer can be dis- 

 tinguished (Fig. 108 A) which, near the stalk, passes into the wall 

 of the perivisceral cavity. The inner mass of the nucleus at this 

 stage consists almost exclusively of the rudiment of the ovary (ov) ; 

 but in the stalk there are some mesoderm-cells which are of 

 importance in connection with the development of the testes, the 

 musculature, the connective tissue, &c. 



Delamination next takes place in the ectodermal layer of the 

 nucleus, which divides into two layers that shift apart (Fig. 108 B). 

 Into the space between the two layers, some of the mesoderm-cells 

 just mentioned wander to yield the musculature of the mantle. The 

 two layers of ectoderm which have thus arisen are known as the 

 outer (am) and inner (im) mantle-layers, on account of their relation 

 to the future mantle of the Sacculina. A second similar process of 

 delamination now takes place in the inner mantle-layer, an inner 

 ectodermal layer surrounding the central part of the nucleus being 

 thus split off. The latter, as surrounding the future visceral sac, is 

 distinguished as the perivisceral ectodermal layer (pe). Between it and 

 the inner mantle-layer there now appears a cavity lined with chitin 

 (Fig. 109, b); this is the so-called brood-cavity (cavite incubatrice). 



