Ascidiacea 561 



and especially pieces of sheet-iron and slate, that lie scattered on the 

 sandy and rocky bottoms west and northwest of Fort Jefferson (Tor- 

 tugas), over which the water at low tide has a depth varying from about 

 1 to 5 feet. These species incubate their eggs in the atrial cavity and 

 give birth periodically to fully developed, free-swimming larvae. 



These compound ascidians are well suited to study in that mature 

 colonies taken from their natural habitat thrive in a live-car anchored 

 near shore and may be transferred daily to aquaria in the laboratory 

 for several hours without injury. 



The dimensions of these larvae, which belong to the Botryllus type 

 in structure and behavior, are as follows: 



Total length Body length Tail length 



Symplegma viride 1.55 mm. 0.40 mm. 1.15 mm. 



Polyandrocarpa tincta 1.13 0.27 0.86 



Polyandrocar pa gravet 1.35 0.30 1.05 



When larvae of these species were desired, a colony from the live-car 

 was transferred to a large glass bowl of fresh seawater in the laboratory, 

 usually between 7 and 8 o'clock in the morning, and placed on a table 

 before a window facing west. At frequent intervals the surface of the 

 water nearest the window was scanned for swimming larvae, for as 

 liberated they accumulated at this most illuminated part of the bowl 

 and were easily collected by means of a wide-mouthed pipette and trans- 

 ferred to beakers or shell vials. 



A large colony of Symplegma viride might usually be relied upon to 

 liberate larvae almost as soon as it had been placed in the bowl and to 

 continue to set them free in a more or less constant stream throughout 

 the day, but unfortunately this species is rare at the Tortugas. Both 

 species of Polyandrocarpa are abundant. They tend to liberate larvae 

 in a swarm once only during the day, usually between 11 a.m. and 

 12.30 p.m., but in one instance a swarm was released as early as 8:30 a.m. 

 and in another as late as 3 p.m. A large mature colony will sometimes 

 liberate swarms of larvae on two consecutive days but none was known 

 to produce a swarm three days in succession. The liberation of a brood 

 or swarm of Polyandrocarpa larvae is a continuous process covering a 

 period varying from about 20 to 40 minutes. The number of larvae 

 constituting a swarm varies greatly with the size and maturity of the 

 parent colony, from less than 50 to more than 1,000, a swarm of average 

 size numbering about 300. (Grave, 1935.) 



PHALLUSIA NIGRA 



The large, black Phallusia nigra is found sparingly on many of the 

 bottoms in the Tortugas region attached to coral rock and to the stems 

 of gorgonians but is abundant on the brick walls within the moat at 



