352 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Carpenter has noted that in those species in which the ambulacra are plated the 

 sacculi are lodged between the successive side plates, the anterior edges of which 

 are notched for their reception, while they occupy small pits in the large plates 

 which cover the genital pinnules. 



In general, it may be said that sacculi are more abundant in shallow than in 

 deep water species, and they are of more uniform occurrence in the Macrophreata 

 than in the Oligophreata. They api^ear to be absent from the species of the family 

 Comasteridse, excepting only Comatonia aetata. 



Bury thus describes the structure of the sacculi. Each sacculus is a spherical 

 body bounded by a thin membrane in which are embedded a few flattened nuclei. 

 The interior of the sphere is more or less filled with a number of pyriform sacks, 

 each of which is, in its turn, filled with highly refractive spherules and is bounded 

 by a membrane continued at its inner end into a long thread which joins the wall 

 of the sacculus. 



The refractive spherules have, at any rate in some cases, a definite arrange- 

 ment around a central cavity. The spherules, which are spherical when isolated, 

 are evidently much compressed within the sacks. 



The greater part of the sacculus is embedded in the tissues, but at one point 

 its cavity is only separated from the exterior by its limiting membrane, which 

 here does not exhibit any nuclei. 



During life the sacculus has a swollen appearance, and its free margin fre- 

 quently bulges out toward the exterior; this is due to the presence of a quantity 

 of mucus between the pyriform sacks and the wall of the sacculus and, if a living 

 piimule be placed on a slide with a drop of water, this water becomes charged with 

 mucus, though whether this is due to a continuous secretion or to injury Bury was 

 unable to say. But any rough treatment of the pinnule leads to the escape not 

 only of increased quantities of mucus but also of many of the spherule-bearing 

 sacks. In spite of this it is rare to find in sections a sacculus freely open to 

 the exterior, and therefore Bury supposes that there is some means of rapidly 

 mending the ruptured membrane. 



According to Reichensperger's observations the matter extruded by the rupture 

 of the sacculi in the living animal consists exclusively of granules in clusters 

 which, as noted by Perrier, much resemble tlie egg clusters. There is no mucus 

 associated with them at any time — a fact which is made clear by the entire absence 

 of a reaction to mucus stains. 



The sacculi of the disk, arms, and pinnules discharge their contents freely into 

 the surrounding medium; those which lie in the wall of the gut, especially at the 

 posterior end, for the most part discharge into the gut. After the periodical 

 rupture of the wall of the sacculus and the sudden extrusion of the granular contents 

 the healing of the wound resembles the healing of the orifice in the pinnule wall 

 through which the eggs are extruded. In most cases in the normal Antedon after 

 the discharge of the contents the capsule of the sacculus quiclfly shrivels, apparently 

 under pressure from the surrounding tissues and from the immediate regeneration 

 of the torn epithelium. Reichensperger was unable to find out what becomes of 

 the remains of the capsule. In the place of the spent sacculus, or in the immediate 



