[341] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 47 
In aquaria, under favorable circumstances, the eggs hatch in about six 
weeks, but in their natural conditions they probably hatch sooner than 
this; under unfavorable conditions the hatching may be delayed for a 
whole year. The eggs are very numerous. In addition to the interest- 
ing observations of Mr. Lockwood, Dr. A. 8. Packard has since given 
more detailed accounts of the development of the embryos and young of 
Limulus in the proceedings of the American Association for the Ad- 
vancement of Science, 1870, p. 247, and in the Memoirs of the Boston 
Society of Natural History, vol. ii, p. 155, 1872. 
Annelids are quite numerous on the sandy shores where the condi- 
tions are favorable. It is evident that these soft-bodied creatures would 
be quickly destroyed by the force of the waves and the agitation of the 
sand, were they not provided with suitable means for protecting theim- 
selves. This is effected mainly in two ways: the sand-dwelling species 
either have the power of burrowing deeply into the sand with great 
rapidity, or else they construct long durable tubes, which descend deeply 
into the sand and afford a safe retreat. . Many of the active burrowing 
species also construct tubes, but they usually have but little coherence 
and are not very permanent, nor do they appear to be much relied on 
by the owners. There is, however, great diversity both in the structure 
and composition of the tubes of different species, and in the modes by 
which the rapid burrowing is effected. 
The large green Nereis (N. virens, p. 317) is found on the sandy 
shores in places that are somewhat sheltered, especially if there be an 
admixture of mud or gravel with the sand to give it firmness and 
solidity. This species burrows deeply beneath the surface and lines 
the interior of its large irregular burrows with an abundant mucus-like 
secretion, which gives smoothness and some coherency to the walls, 
but does not form a solid tube. With this, and in greater numbers, the 
smaller species, Nereis limbata, (p. 318,) is also found, and its habits 
appear to be essentially the same. Both this and the preceding can 
burrow rapidly, but much less so than some other worms, and conse- 
quently they are not well adapted to live on exposed beaches of moving 
sands, but prefer coves and harbors. The two large species of Rhyn- 
chobolus are much better adapted for rapid burrowing. Their heads 
are very small and acute, and destitute of all appendages, except four 
minute tentacles at the end; the body is long, smooth, and tapers 
gradually to both ends, and the muscular system is very powerful, and 
so arranged as to enable these worms to coil themselves up into the shape 
of an open spiral, like a corkscrew, and then to rapidly rotate them- 
selves on the axis of the spiral. When the sharp head is inserted into 
the loose mud or sand and the body is thus rotated, it penetrates with 
great rapidity and disappears almost instantly. Both these species are 
found on sandy as well as on muddy shores and flats near low-water 
mark, and also in deeper water. The one usually most abundant is 2. 
dibranchiatus, (Plate X, figs. 43, 44;) this is readily distinguished by hav- 
