l8o MORPHOLOGY OF INVERTEBRATE TYPES 



tern. The ectoneural nervous system follows the plan of the am- 

 bulacral and circulatory systems. It is epidermal in origin, be- 

 longs to the ventral surface of the animal and is situated below 

 the system of sinuses covered only by the epithelium of the body 

 wall. Its function is sensory. It consists of a nervous ring sur- 

 rounding the peristomial membrane, five radial nerves which give 

 off transverse nerves and a peripheral network innervating the feet 

 and the pedicellariae. The hyponeiiral or motor system is struc- 

 tured like the ectoneural from which it is separated only by a 

 very thin membrane of connective tissue. The entoneural system 

 belongs to the dorsal ccelomic epithelium and consists of five 

 nerves radiating from the centre. 



Reproductive system. The sexes are separate, but the 

 structure of the reproductive organs is similar in both sexes. 

 Five pairs of ramified adradial gonads open to the outside through 

 as many genital pores which are interradial and dorsal in their 

 position. When fully developed the gonads extend far into the 

 arms, on each side of the digestive cceca. 



The eggs are fertilized in the water. The development is a 

 metamorphosis in which the larval stage is represented by the 

 Bipinnaria. 



Instructions 



1. (a) Dry specimen. Make a full size drawing of the aboral 

 or dorsal surface of the starfish showing the central disc and all 

 arms, madreporic plates, tubercles and pedicellarias. Indicate 

 by a dotted line the plane of symmetry and label bivium and 

 trivium. The anus is usually not visible but should be indicated 

 by a dot in the centre of the disc. The details should be drawn 

 only in one arm. 



2. Remove with a forceps the ambulacral feet of one arm. 

 Make a full size drawing of the oral or ventral surface, showing 

 mouth, oral spines, ambulacral grooves with four rows of am- 

 bulacral pores between the ambulacral ossicles, spines on the 



