i8o 



THE INVERTEBRATA 



The mode of development described above is typical in the Scypho- 

 medusae. There are, however, certain exceptions. In the genus 

 Pelagia the medusa develops directly from the egg into an ephyra 

 larva, and in Cassiopeia the hydratuba only produces a single ephyra 

 at a time, a condition which is obviously primitive compared with 

 Aurelia, "polydisc" strobilation being a secondary adaptation for the 

 more effective spread of the species. 



Fig- 135- Diagrammatic longitudinal section through Pilema. Enteron and 

 its branches shown in black, many "sucking mouths" along the lips, can.r. 

 radial canal ; sg.p. subgenital pit. 



Class ACTINOZOA (ANTHOZOA) 



Solitary or colonial coelenterates with polyp individuals only: 

 coelenteron divided by mesenteries: stomodaeum present: genital 

 cells derived from endoderm. 



They are divided into the two orders Alcyonaria and Zoantharia. 



Order ALCYONARIA 



Actinozoa with eight mesenteries and eight pinnate tentacles ; stomo- 

 daeum with a single siphonoglyph (ciliated groove) ; skeleton internal, 

 consisting of spicules in the mesogloea, occasionally supplemented 

 by an external skeleton ; longitudinal muscles on the ventral faces of 

 the mesenteries. 



As a type of the order we will describe Alcyonium digitatum, *' Dead 

 men's fingers", a colonial form which occurs below low-tide mark, 

 attached to stones, in various sizes and shapes, but usually in broad- 

 lobed masses. A small portion or lobe of a colony is shown in Fig. 136, 

 and it is seen that the polyps project in life from the general surface 

 of the colony. The ectoderm, mesogloea and endoderm of the polyps 



