20 STRUCTURE OF THE CYSTIDE^. 



communicate with the interior through the mouth, one of the 

 functions of the aperture is that of an ambulacral orifice. In those 

 species without an anus, the ova and excrements are extruded 

 through the oral opening. In many which have an anus there are 

 several sets of genital pores for the production of the eggs. 



The following then are the apertures which in the Star-fish with- 

 out an anus are all combined in one : — 



1. The mouth. I 3. The ovarian apertures. 



2. The anus. I 4. The ambulacral orifice. 



In those with an anus and genital pores, the mouth has of course 

 but two functions. 



II. Recent Crinoids. 



In the recent species of Crinoids, such as Pentacrinus caput-Medmce, 

 and the several species of the genus Comafula, the ambulacral 

 grooves radiate from the mouth and are continued along the ventral 

 sides of the arms to their extremities. At first there are five only, 

 but these divide into ten before reaching the margin, in order to 

 furnish a groove for each of the ten arms (see fig. 1). The grooves 

 also send off slender branches to the pinnulce, and they are all 

 provided with suctorial feet, ambulacral canals, nerves and blood- 

 vessels, as in the Star-fishes. The young however do not escape 

 through the mouth, but the ova are developed beneath the soft skin 

 in the grooves on the ventral side of the pinnulae, and when the 

 proper time arrives are set free. The vessels of the arms all enter 

 tiirough the mouth, and as there is always a distinct anus, it follows 

 that this aperture has two functions. 

 1st. It is the mouth. 

 2d. It is the ambulacral orifice. 



Sd. It must also, to some extent, be regarded as an ovarian 

 aperture, because in all animals there must exist a connection of 

 some kind between the reproductive and nutritive systems. It has 

 been demonstrated by Mr. J. V. Thompson of Cork (see Edinburgh 

 New Philosophical Journal, 1836), that the ova, as above mentioned, 

 are generated in the arms and pinnulee outside of the body ; and 

 as there is no other aperture for that purpose, then it must follow 

 that whatever may be the vessels by which a communication is 

 effected between the ovaries and the interior, they can only pass 

 through the mouth. For the same reason, the ambulacral orifices 



