INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 341 



t 

 In aquaria, under favorable circumstances, the eggs batch in about six 

 ^yeeks, 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. S. Packard has since given 

 more detailed accounts of the development of the embryos and young of 

 Limulus in the proceeditigs 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 them- 

 selves. Tills 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. Manv of the active burrowiuir 

 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 i)laces 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 Umhata, (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 Rhyii- 

 choholus 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 R. 

 dibranchiatus, (Plate X, figs. 43, 44;) this is readily distinguished by hav- 



