498 INVEETEBEATA CHAP. 



Ophiuroid. In the Asteroid, as we have seen, the dividing wall 

 between the peri-oral and the left posterior coelom breaks down and 

 leaves as remnants the ten " retractors " of the stomach. The 

 intestine and larval arms have by this time completely disappeared. 



From the walls of the radial perihaemal spaces proliferations of 

 cells take place. Those on the ventral side of these spaces are 

 apposed to the thickened bands of ectoderm which form the radial 

 nerve cords, and they constitute the coelomic ganglia, which are the 

 motor elements in the nervous system. From the dorsal walls of 

 these spaces masses of cells are given off which form the great 

 longitudinal muscles connecting the " vertebrae " of the arm. From 

 the sides of the arms movable hooks are developed (h.k, Fig. 379). 

 These hooks are found, in the adult Ophiothrix, ventral to the 

 transverse rows of arm spines, and are characteristic of the genus. 



By this time the metamorphosing larva has reached the bottom, 

 and it commences to walk on its tube feet. The postero-lateral arms 

 shrink, the flesh retreating to their bases and the spines becoming 

 exposed. The naked spines are soon broken off and the young brittle- 

 star walks away. Very soon its arms have become so long that the 

 wriggling movements, so characteristic of the adult, supersede the 

 action of the tube feet. 



AMPHIURA SQUAMATA SKELETON AND GENITAL ORGANS 



The development of the calcareous skeleton and of the genital 

 organs has not been followed out in Ophiothrix, but has been worked 

 out in the species Amphiura squamata. As mentioned above, this 

 animal carries the young about in its genital bursae until they 

 resemble the parent in all points except size and the development of 

 the genital organs. It is hermaphrodite, one testis and one ovary 

 discharging into each bursa. 



Eusso (1891) has given a general account of its development, but 

 as this worker did not employ the method of sections to any great 

 extent, and as the young stages are met with seldom and are very 

 opaque, it is quite likely that his account is inaccurate. According to 

 Eusso the blastula is converted into a two-layered gastrula by delami- 

 nation (!), each cell dividing into an inner endodermic and an outer 

 ectodermic portion, and the coelom is said to arise as splits in 

 a mass of mesenchyme. Such statements as these are improbable in 

 the highest degree. The embryo is oval without any of the outgrowths 

 characteristic of the Ophiopluteus ; it has, however, a larval skeleton, 

 consisting of a network of calcareous trabeculae, which is absorbed 

 before the metamorphosis is complete. This network arises as two 

 calcareous " stars " which arise in the mesenchyme to the right and 

 left of the alimentary canal, and which branch, and their branches 

 unite to form the network. Even before the pentagonal form is 

 attained the rudiments of the adult plates begin to appear. 



The most satisfactory account of the development of these plates 



