388 /OOLOGY 



SECT. 



those on the oral surface, are for the most part more prominent, 

 so that they assume the character of short spines. The ossicles 

 on which they are borne are star-shaped with six rays, a 

 spine being borne in the centre of each ossicle, and one on 

 each of the rays. Between the ossicles the surface is covered 

 with a soft, slimy skin, perforated by a large number of minute 

 dermal pores, each of which is enclosed by a minute irregular ring of 

 calcareous matter ; each pore serves for the lodgment of one of the 

 dermal branchiae. Numerous pedicetlarise, similar to those on 

 the ventral surface, but smaller, are borne on the ossicles, usually 

 taking the place normally occupied by the central spine. The 

 tube-feet are arranged in a single row on each side of each ambu- 

 lacral groove ; but the ampuUce are in two rows, an upper and a 

 lower, and each tube-foot has two ampullae connected with it, 

 one of the upper row and one of the lower row. 



Anthenea has vertical calcareous inter-radial partitions not de- 

 veloped in Asterias. There are five bifid intestinal cceca, which 

 are narrow tubes slightly enlarged and lobed at the extremities. 



Development of a Starfish (Asterina gibbosa or A. 

 exigua *). In these Starfishes the reproductive apertures are 

 placed on the ventral surface. When the ova have been dis- 

 charged and impregnated, they adhere by means of a viscid 

 investment to the surface (rock or stone) on which they are laid, 

 and go through all the stages of their development in this position, 

 never passing through a free pelagic stage. The eggs are about 

 half a millimetre in diameter, and of a spherical shape. Each con- 

 sists of a perfectly opaque central mass of yellow or orange yolk, 

 and of a glassy layer enclosing this. After fertilisation the process 

 of segmentation begins by the division of the ovum into two blasto- 

 meres almost equal in size, but one, which may be termed cell 

 I., slightly smaller than the other (cell II.). Both I. and II. soon 

 afterwards divide, I. somewhat earlier than II. The resulting 

 four cells again divide, leading to the formation of an eight-celled 

 stage (Fig. 313, A), in which the four cells derived from I. form 

 an incomplete ring not closed below, and the four derived from 

 II. form an incomplete ring open above. 



The eight cells then divide by meridional fissures into 

 sixteen, and a further division results in the formation of thirty- 

 two. The thirty-two cells become arranged in such a way as to 

 enclose a central cavity which had been present in the four-celled 

 stage : this stage (B) is the blastula ; the cavity is the segmenta- 

 tion-cavity or Uastoccde. The number of cells in the wall of this 

 cavity increases by further divisions, and the whole surface becomes 

 covered with vibratile cilia. A process of imagination then 

 follows, one side of the blastula being pushed inwards to form 



1 The development of these has been described in preference to that of the 

 examples, as it is more completely known. 



