Introduction to Animal Morphology. 1 5 1 



stiff, inflexible wall, composed of an outer and inner 

 porous, and a middle spiculigerous membrane, and 

 the cavity may be labyrinthine. In the Dipneumona, 

 five branches pass to the calcareous ring, on the inner 

 side of which they divide into branches to each ambu- 

 lacral foot and tentacle. The pedicelli are conical, 

 without, or cylindrical, with a sucking disc, sometimes 

 irregularly scattered (sporadipoda), or in 2-5 rows 

 (stichopoda), making a discriminable dorsal (bivial) 

 and ventral (trivial) surface. The abdominal feet may 

 differ from the dorsal (heteropoda tristicha) ; rarely 

 the bivial fail altogether, and the ventral are limited 

 to a few series on the abdomen (Psolus). The pedicelli 

 have rudimental rosette plates ; some non-retractile 

 feet have calcareous supports in their side walls. 

 Progression is by creeping ; the actinoid Minyas 

 walks on its tentacles, and some Synaptae can swim. 



Except in Synapta and Molpadia, the sexes are 

 separate, as in the other Echinoderms. The sex- 

 organs are not radial, but single ramose, attached to 

 one or both sides of the dorsal mesentery. There is a 

 common duct opening dorsally beneath the mouth. 

 Only when the products are ripe can the testis and 

 ovary be distinguished from one another. The zoo- 

 sperms are pin-shaped ; the ovum has a firm, radially- 

 striated envelope, with a micropyle ; Synapta vivipara 

 is viviparous ; Holothuria tremula develops directly ; 

 the others mostly undergo metamorphoses. From the 

 Q^% escapes a bilaterally symmetrical planula, which 

 develops an anus and a digestive canal. The cilia 

 limit themselves to two girdles, and the form is then 

 called Auricularia, with short sac-like processes, some- 

 times spicules, but no calcareous skeleton. In moving. 



