358 ZOOLOGY SECT. 



opposite each of the intervals between the arms. The surface is 

 dotted over with numerous small rounded tubercles, arranged in 

 somewhat irregular radiating lines. These dorsal tubercles, though 

 fewer than those on the ventral 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 pedicellariae, 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 ampullce are in two rows, an upper and a 

 lower, and each tube-foot has two ampullae, one of the upper 

 row and one of the lower row, connected with it. 



There are vertical calcarious inter-radial partitions not de- 

 veloped in Asterias. There are five pairs of intestinal cccca, which 

 are narrow tubes slightly enlarged and lobed at the extremities. 



Development of a Starfish (Asterina gibbosa or A. 

 exigua 1 )- In these Starfishes the reproductive apertures are 

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

 charged aiid impregnated, they adhere by means of a viscid 

 matter 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, the result being the formation of an eight-celled 

 stage (Fig. 287, 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 olastoccele. The number of cells in the wall of this 



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

 examples, as it is better known and more readily studied. 



