56 University of California Publications. Tzoology 



the medusoids, which are motionless on the free ends of narrow 

 blastostylar processes with no perisarcal covering. 



I have seen neither tentacles on, nor fertilized ova in, intra- 

 capsular medusoids, though a great many observations have 

 been made. In rare instances, an extracapsular medusoid is 

 found with unfertilized ova. On one occasion a dancing mass of 

 sperm was crowded around the bell mouth of an extracapsular 

 medusoid, apparently attracted to that spot. It is here that the 

 sperm probably penetrate, after the medusoid has left the gono- 

 theca. 



There may be four embryos in each female medusoid, which 

 are retained until the planula stage. 



There are usually four medusoids to the blastostyle in female, 

 and five to seveu in male colonies. Occasionally blastostyles 

 with sterile medusoids are met with, such as those described by 

 Weismann ('83). The cause of this sterility has not been 

 determined, so far as I know. It is the more obscure because a 

 whole colony is not always affected, biit only here and there a 

 blastostyle. The general external conditions of temperature, 

 oxygen, light, food, etc., would seem to be the same for all 

 parts of the colouy; so that the cause is probably local, possibly 

 malnutrition from some mechanical defect in the circulatory 

 canals. Colonies in the University of California collection with 

 medusoids taken in January, March, April, May, September. 



Obelia. 



Trophosome. All kuown species branched; otherwise as in Canipanularia. 



Gonosome. Blastostyles in axils of branches, giving rise to free disk- 

 shaped medusae with four radial and a ring canal, eight lithocysts and more 

 than eight tentacles; mouth without tentacles. 



Obelia commissuralis, McCr. 



Obelia commissuralis, McCrady, Gymno. Charls. Harbor, p. 95. 

 Agassiz, Contr. N. H. U. S., 1862, IV, p. 315. 



Distribution. San Francisco Baj r , Cal., between tides. 

 Eastern U. S. (Agassiz). 



This species has not been recorded previously for the Pacific 

 Coast. The San Francisco specimens are identical with Agassiz 's 

 description of the Eastern form. 



