120 



LOWER INVERTEBRATES. 



Fig. 114. — Aetrcea pallida. 



mentioned, in which lies the stomach. Manidna is commonly found on the floor of 

 a coral lagoon, but is not attached to the bottom. Here also belongs the genus 

 Seliastrcea. 



Several other genera of Actinoida assume, in the manner of their growth, the shape 



of hemispherical heads, although 

 they do not have a convoluted 

 surface like the true brain corals. 

 One of the best examples of these 

 is Astroea, a coral in which the 

 colony has a globular form, but 

 without superficial canals. The 

 different coral individuals in 

 Astrcea are placed side by side 

 over the whole surface without 

 being arranged in lines, while 

 every individual is externally 

 almost wholly distinct from 

 every other in the colony. The 

 mode by which animals of this genus reproduce their kind, and increase the size of 

 the community is by simple seK-division or fission. When the single individuals, by 

 their growth, exceed a certain size, they spontaneously divide into two similar well- 

 formed smaller individuals. 



The only Actinoid coral which secretes a stony base found in New England waters 

 is a beautiful little genus known as Astrangia, 

 which occurs in small patches in the crannies 

 and clefts in the cliffs along the southern shore 

 of New England. At Newport, R. I., on the 

 most southern point of the island, there are sev- 

 eral localities where beautiful colonies of these 

 animals are found. This is, however, but one 

 of many localities which might be mentioned. 

 The calcareous base which the Astrangia se- 

 cretes is inconsiderable in size, and forms only 

 a slight crust on the surface of the rocks. 

 It builds no considerable reefs or coral de- 

 posits of any size. The animal itself is one 

 of the most delicate and beautiful of all the reef-building corals. 



No living genus of corals better illustrates the formation of new individuals by self- 

 division than that known as Mussa. Here we generally have a limited number of 

 individuals, never branching, but attached to each other at their bases. Almost every 

 fragment of one of these corals shows individuals with evidence of a self-division, 

 either just beginning, partially formed, or wholly completed. In the parent individual 

 before any sign of division begins, the upper extremity or distal end is of circular form 

 and the coral itself has an irregular trumpet-like form, fastened by the smaller end to 

 some foreign object. The first sign of change in the original Mussa preparatory to a 

 fission, is the elongation of the disk-like shape or a lengthening of its axis, by which 

 the two opposite sides closely approach each other. This growth ultimately leads to 

 a condition in which the two opposite sides approach, and the disk of the coral is 



Fig. 116. — Astrangia dance. 



