Summit Slnicturc of J^enlrc miles. 485 



The structure of the summit covering of P. Norwoodii, 

 just described, would seem to render it quite improbable 

 that the central aperture was the mouth ; which being the 

 case, we are obliged to consider the aperture usually re- 

 garded as the anal opening as also the buccal aperture, 

 and in addition to this, in this species, it must have also 

 served as the vent of two of those canals which end in 

 the so-called ovarian openings ; which arrangement would 

 seem to be more complicated than their known position 

 in the scale of organization would seem to indicate ; and 

 to regard those summit openings as ovarian apertures does 

 not seem consistent with the knowledge we have of the 

 mode of deposit and dispersion of the ova of crinoids, 

 with which these animals are so closely allied. 



It seems at least probable that the so-called ovarian 

 apertures performed a far simpler, but scarcely less im- 

 portant function, — that of apertures of siphonal tubes for 

 the vibration of the tentacula, by the injection and expul- 

 sion of water ; communicating, by way of the tubular 

 apparatus beneath each pseudambulacral field, with each 

 tentacle through the pores, expanding them by the infla- 

 tion of the tubular canal along their inner grooves, and 

 contracting them by the expulsion of the water. It is 

 conceived that this operation of the siphonal tubes would 

 produce all the necessary motion of the tentacula, and be 

 more in accordance with the status of the animal than 

 a complex muscular system, which would otherwise 

 be necessary to operate the hundreds of those minute 

 organs. 



Moreover, if the tubular openings of this species are to 

 be regarded as ovarian apertures, it will be necessary, as 

 before remarked, to disregard the close analogy of these 

 animals with the true crinoids, in which the ova are de- 

 veloped in little accessory sacs at the base of each ten- 

 tacle, between which organs and the summit tubes of this 



