254 



PROF. A. C. HAD DON ON TWO SPECIES OF 



Eig. 6 represents a section through the tertiary mesentery 

 indicated by an asterisk ( # ) in fig. 5. The secondary and ter- 

 tiary mesenteries alone are fertile. The total number of me- 

 senteries is 96 (12 + 12 + 24+48). 



The invaginated ectoderm consists of a ciliated columnar epi- 

 thelium, that on the tentacles is crowded with curved nematocysts 

 (fig. 7) ; granular gland-cells are common in the epithelium of 

 the oesophagus. Where it is preserved, the ectoderm of the 

 column is coated with an investment of grains of sand appa- 

 rently imbedded in a mucous matrix often with shreds of 

 cuticle ; on the pedal disk, however, the cuticle is strongly de- 

 veloped between the arenaceous investment and the epithelium. 



The character of the mesoglcea (mesoderm) is sufficiently indi- 

 cated in fig. 10. This figure also illustrates the structure of the 

 circular muscle in vertical section. 



The concrements observed by Professor Hertwig in the super- 

 ficial layer of mesoderm in his specimen are here absent; but oval 

 foreign bodies occur in the mesoglcea mainly towards the apex of 

 the invaginated section of the body-wall, either within or outside 

 the circular muscle. They appear to be the ova and embryos 

 of some parasite, possibly of a Nematode worm. 



The endoderm consists of the usual histological elements. The 

 madder-coloured pigment of the tentacles and oral disk is con- 

 fined to the endoderm. In those mesenterial filaments (craspeda) 

 which have a trilobed section, the central lobe only possesses 

 unicellular glands which are of the granular type. 



The acontia (PI. XX. fig. 9), in addition to granular gland- 

 cells, are crowded with nematocysts (fig. 8) ; these are rather 

 longer and more slender than those of the tentacles, the former 

 being about '0083 of an inch, and the latter about '0015 of an 

 inch in length. 



DESCRIPTION OF THE PLATES. 



Plate XIX. 



Myriactis iubicola, gen. & sp. nov. 



h. Craspeda (mesenteric filaments), d. Unicellular glands, en. Endoderm. 

 (). Reproductive organs (gonads), h. Mesenteries; I.-IV. indicate the rank of 

 the pairs of mesenteries, i. Oral disk. k. Body-wall. I. Pedal disk. me. Mo- 

 sogkea (mesoderm), ml. Longitudinal muscles of mesentories. mp. Parioto- 

 basilar muscle, ms. Circular muscle. 0. Ova. e. (Esophagus, .sc. Suckers. 

 ur. (Esophageal groove. I. Tentaclee. z. Zooxanthcllu;. 



