98 
surface of about 0.19 mm. The ovum meas- 
ured about 0.08 mm. (Fig. 20 a, b, c). Devel- 
opment of the ova was not investigated. 
It is of interest to compare the egg cap- 
sules and ova of this species with those of 
two other species of the genus from Bermuda, 
which have been recorded by Dr. Marie V. 
Lebour. They are L. zigzac Dillwyn ( Lebour, 
1945) and L. neritoides (Lebour, 1935). 
Order OPISTHOBRANCHIATA 
Suborder tectibranchiata 
Family APLYS1DAE 
Genus Tethys (= Aplysia) 
Tethys (= Aplysia) grandis (Pease) 
Fig. 21 
On November 9, 1920, two egg filaments 
of Tethys grandis were found beside the Ma- 
rine Laboratory pier adhering to a stone un- 
der which were two of the animals. Since 
then numerous filaments have been seen on 
the loose stones at the same place, to which 
the mollusks come in large numbers. They 
were especially abundant during the months 
of November, December, and January, which 
seemed to be the main spawning season. 
They were also present at other times of the 
year, in August for example, and their spawn 
would invariably be found with them. 
The filament of this mollusk resembled a 
very tangled bundle of yellow thread with a 
circumference of about 15 cm. and a thick- 
ness of about 3 cm. It was supplied with thick 
walls and contained masses of oval or rounded 
hyaline membranes packed together through- 
out its length. These hyaline membranes lay 
3 or 4 abreast in the filament and each con- 
tained 7 to 15 ova (Fig. 21^ b) . 
Early stages of cleavage were investigated 
in different filaments of the species. In one 
instance, the first cleavage was equal, the 
second unequal and resulted in two smaller 
blastomeres asymmetrically placed. This 
would indicate spiral cleavage. The ovum 
measured about 0.085 mm. (Fig. 21c-/). 
PACIFIC SCIENCE, Vol. IV, April, 1950 
0 I 2 3 cm. 
Fig. 21. Tethys (= Aplysia) grandis (Pease). 
a, Egg filament; b 9 magnified section of filament 
with active embryos; c—f, first type of cleavage; 
g—i, second type of cleavage; j, k, shell of free- 
swimming veliger stage showing operculum (j); 
l, operculum more highly magnified. 
