164 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



opening 



,Plc.. 255. 



A, Pentremites iiyriformis Ba,y. Portion 

 of an ambulacrum, exhibiting the lancet- 



considerably complicated in structure (Fig. 255). The centre of each ambu- 

 lacrum is occupied by the lancet-plate, a long, narrow piece, pointed at both 

 ends, which extends to the full length of the fields. Its proximal end is 

 inserted between the deltoids, and takes part in the lip around the summit- 

 The upper surface of the plate is excavated along the median line, 



and forms an open, well-defined groove, which 

 conducts to the mouth, and in all probability 

 represents the food-groove. The interior of the 

 plate is traversed by an axial canal, which com- 

 municates by means of the ambulacral opening 

 with an oral ring belonging to the water-vascular 

 system. In a number of forms (Pentremites, 

 Orophocrinus) there is to be seen a second, 

 smaller, and extremely thin plate underlying, 

 the median portion of the first ; this is called 

 the under lancet-plate. 



The lancet-plate rarely occupies the full 



width of the ambulacral field, and the spaces 



between its lateral edges and the sides of the 



radial sinus are either wholly or partially 



i)iate (I) ; median food-groove of the same covcred bv a row of Small, horizontally elongated 



(rt) ; side-plates (s) ; outer side-plates («) ; i , /a 7 j » j; -r> \ t D ^ 



and marginal pores (p), 5/1, (after E. and sule-plates {" pore-puites ot Koemer). in rent- 



0. B, Ambulacrum of Nucleocrinus. -i rt i ■ i ii u; 



Lettering as in ^ (after Roemer). remites, Oropkocnnus, and Other genera, an addi- 



tional series of still smaller pieces, called the 

 outer side-plates ("supplementary pore-plaies" of Koemer), are placed between 

 the side-plates and the walls of the radial sinus. Pentremites and Cryptoschisma 

 have the entire upper surface of the lancet-plate exposed to view, and the 

 side-plates are situated alongside of it in the 

 same plane. But in other forms the lancet-plate 

 is wholly, or to a very large extent, concealed by 

 the side-plates (Fig. 255, B), so that as a rule 

 only a small space along the food-groove is visible. 

 The sutures between the side-plates are indicated 

 by shallow, horizontal furrows, which are con- 

 tinued as superficial grooves over both halves of 

 the lancet-plate as far as the median ambulacral 

 groove. These crenulations, it should be noted, 

 are frequently efi"aced in weathered specimens 

 (Fig. 255, A and B). Small, pit-like depressions, 

 or small tubercles, which are observable on the 

 side-plates, indicate the places where the append- m;;^-^;""^ °LSrS''and"S^t.; 

 ages or pinnules were formerly attached. These are perfectly preserved pinnules (after Meek 



o J . and Wortlien). 



only exceptionally found intact, but when pre- 

 served they completely conceal the ambulacral fields, and extend upward above 

 the summit of the calyx (Fig. 256). They differ considerably in length, even 

 among species belonging to the same genus. They are jointed structures like 

 miniature arms, uniserial as far as observed, but with ossicles sometimes 

 wedge-shaped and interlocking to some extent from opposite sides, thus 

 simulating a biserial arrangement. Whether they performed the function of 

 discharging the ova, like the pinnules of Crinoids, can only be conjectured. 



