232 CCELEXTERATA 



compact masses. Astrangia dance (fig. 203), only coral in New England; Astrcea; 

 brain corals (Ccvloria, fig. 204, Mairicina); Favia (fig. 205). (2) FUNGIACEA, 

 or mushroom corals, no theca. Some colonial, others (Fungia) solitary. A 

 sort of strobilation in development. (3) POROSA, with skeleton porous like a 

 fine sponge. Madrepora* deer's-horn coral (fig. 206), Poritcs, Astroides. 



FIG. 206. Madrepora erythrcca (after Klunzinger). 



Class IV. Ctenophora. 



The Ctenophores excel all animals, even the medusa?, in transparency 

 and delicacy of tissues; many are so soft that a strong current tears them, 

 and no attempts to preserve them have been successful. The body is 

 biradially symmetrical; i.e., is divided by both sagittal and transverse 

 planes into symmetrical halves. Since the longitudinal axis is usually 

 longer than the others, which are generally equal, the body is usually 

 oval or pear-shaped. In Cesium the sagittal axis is greatly longer, giving 

 the animal the form of a band, whence the name 'Venus girdle.' 



The bulk of the animal is composed of a soft jelly with connective- 

 tissue cells, penetrated in every direction by polynucleate muscle cells 

 (fig. 50) branched at their ends and apparently innervated by special 

 nerve cells. On the outer surface is a layer of ectoderm, while in the in- 

 terior is a system of branched entodermal canals. 



At the bottom of a depression (fig. 207, s) at the aboral pole is a thick- 

 ened patch of ectoderm, the sense body, a typical statocyst (fig. 208). 

 The thick sensory epithelium forms a shallow groove, strong hairs which 

 rise from the edge of the groove arch over it, enclosing a space to be com- 

 pared to an incomplete vesicle. In the centre is a spherical mass of stato- 

 liths, supported on four bundles of S-shaped agglutinate cilia. From 

 these bundles of cillia eight bands of thickened epithelium, at first in pairs 

 (fig. 209, ws), later diverging, pass to the oral pole (fig. 207, ;-). These 

 meridional bands (so called from their course) consist in part of ciliated 

 epithelium, in part of the characteristic 'combs' which are the locomotor 



