8 



THE HYDROMEDUSAE 



in a direction at right angles to the long axis of the body and to 

 the contractile fibres of the ectoderm. The cells are often 

 amoeboid at the outer or free end, and contain vacuoles filled with 

 an albuminous fluid. Particles of food-matter and masses of (?) 

 excretory matter are often to be detected in the protoplasm. 

 Among these larger cells are often intercalated gland cells, which 

 appear to secrete a digestive fluid. Ganrjlion cells and pi'jment cells 

 occur ; but though nematocysts have been detected in endoderm 

 cells, it is still doubtful whether they are formed in them 

 or not. 



Where they form the axial core of a solid tentacle, the endoderm 

 cells become vacuolated and " cartilaginous " in consistence, re- 

 sembling the notochordal cells of Chordata (Fig. 10). 



The mesofjloea forms a thin lamina everywhere between ectoderm 

 and endoderm cells and gives by its stiff"ness a certain rigidity to 

 the body. It is often apparently laminated. Although itself in- 

 capable of contraction, it is greatly thickened and shortened, on the 

 contraction of the body, by the muscular fibres of the ectoderm and 

 endoderm. 



HiSTOLOCxY OF THE Medusoid. — The ectoderm appears over the 

 greater part of the umbrella as a layer of much flattened cells, but 

 is cubical on the velum and manui)rium. Epifhelio-muscuhir cells, 

 like those of the hydroid, occur also in the medusoid, but sub- 

 epithelial muscle cells are here more common ; they are either 

 scattered, or grouped in traV>eculae, and in some cases become em- 

 bedded in the mesogloea. The ectodermal muscle fil)res may have 

 either a circular or longitudinal trend, unlike those of the hydroid. 



On the manul)rium cir- 

 cular musculature is 

 well developed ; longi- 

 tudinal fibres also occur 

 on it, which are con- 

 tinued cent ri f ugally out- 

 wards, radiating over 

 the subumbral siu"face 

 towards the lip of the 

 bell. The subumbrella 

 ])ossesses also circular 

 fibres ; the exumbrella 

 has little or no muscu- 

 Strongly developed circular fibres characterise the edge 



Fio. 10a.— Muscular cells of medusae (Liszia). The 

 uppennost is n purely muscular cell from the subumbrella ; 

 the two lower are fpitlielio-muscular cells from the biise of 

 a tentacle ; the upslamliu^; nucleated jwrtiou forms part of 

 the epidennal mos;iic uii the free surface of the l>o<ly. 

 (From Lankester, after Herlwij;.) 



lature 



of the bell and the velum ; by their agency the contraction and con- 

 sequent progression of the bell are chiefly eflected. The tentacles 

 are highly contractile, and are provided with strong longitudinal 

 muscles. Sensori/ cells, which are elongated and columnar, and are 

 provided with palpocils, are well developed at the bases of the 



