590 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



A complete perisomic plating is found in Comactinia meridionalis, Comissia 

 littoralis, and in the recently detached young of Thaumatocrinus renovatus and 

 Pentametrocrinus, sp., while the exposed perisome is unplated in 



fLamprometra protectus. Antedon adriatica. 



Ptilometra. millleri. Heliometra (jlac-ialis. 



Crotalometra porrecta, Promachocrinus kerguelensis. 



Antedon petasus. Anthometra adriani. 



Antedon bifida, Hathrometra prolixa. 



Antedon mediterranea. Hathrometra tenellu. 



Hathrometra sarsii. 



In the adults of all of the comatulids the perisome of the disk contains spicules 

 and concretions which are usually quite invisible to the unaided eye, though they 

 sometimes take the form of plates of considerable size, and indeed in the family 

 Calometridse and in certain species of the genus Eudiocrinus the visceral mass is 

 completely inclosed in a rigid calcareous pavement. 



There is no evidence that these calcareous deposits are in any way connected 

 with the larval perisomic plating of the disk, which in the four types in which it 

 has been demonstrated completely disappears long before the full size is reached. 



Spicules of the tentacles. Almost from their first appearance the tentacles 

 contain long more or less curved and knobby calcareous rods of various sizes (part 

 1, pi. 6, figs. 559,563). 



HYBRIDS BETWEEN CEINOIDS AND ECHINOIDS. 



Dr. Emil Godlewski, jr., working at the Stazione Zoologica at Naples, fer- 

 tilized with spermatozoa from Antedon mediterranea the eggs of Echinus micro- 

 tuberculatus de Blainville, Paracentrotus lividus (Brandt) and Sphcerechinus 

 granularis A. Agassiz. This was found to be possible when the alkalinity of 



the sea water was raised by the addition of 0.75-1.25 c. c. r^r (caustic soda). In 



his experiments all precautions were taken in order to avoid the possibility of 

 error through the presence of echinoid spermatozoa and the possibility of partheno- 

 genesis was excluded by control cultures. 



Godlewski found that a certain relation must exist between the two types 

 of genital products. As a general rule, a large amount of sperm must be used 

 for a relatively small quantity of eggs. 



There is considerable individual difference in the animals used for experi- 

 mentation. Other things being equal, one gets a very different percentage of 

 fertilized eggs from different animals of the same species. 



Not only the genital products of different individuals, but also individual eggs 

 and spermatozoa from the same animal vary in a fixed ion concentration. If eggs 



and spermatozoa in sea water with a slight addition of caustic soda, 0.25 c. c. ^ 

 NaOH (caustic soda) : 100 sea water, are passed through mixtures of in- 

 creasingly higher alkalinity 0.50, 0.75, 1.25, and 1.50 c. c. 10 caustic soda ^i 

 100 sea water the percentage of fertilized eggs is considerably raised. 



