THE SEA-SLUGS. 257 



Although Dendronotus can hardly be described as com- 

 mon between tide-marks, I have not infrequently found 

 specimens there. They are, however, usually of small size, 

 while specimens from deep water reach a length of two 

 inches. As is the case with most of the shore inverte- 

 brates, the animals breed long before they attain the maxi- 

 mum size of the species, so that I have had specimens of 

 under an inch in length which laid numbers of eggs in 

 confinement. Facts of this kind are very apt to puzzle 

 novices accustomed to land animals, whose life is more or 

 less sharply divided into two parts an early period of 

 growth, and an adult period of reproduction. It should 

 be clearly understood that such a condition of affairs is rare 

 among marine invertebrates, which have usually no definite 

 limit of growth, and which begin to reproduce very early. 

 The result of this is that statements as to size are often 

 very deceptive, for the limit given is usually that observed 

 by some authority on the particular group, and the animals 

 of the area at your disposal may show great variation as 

 compared with this standard. Thus in the Firth of Forth 

 the common starfish grows to a size much larger than the 

 limit usually given, especially when it occurs in the vicinity 

 of extensive mussel beds. On the other hand, in many 

 cases the sea-slugs which congregate for breeding purposes 

 are all distinctly below the standard of size as determined 

 for other areas. It is not perfectly clear why marine in- 

 vertebrates should differ so markedly in this respect from 

 terrestrial forms, but there is no doubt that on the whole 

 the conditions of life are easier on sea than on land. The 

 high specific gravity of sea-water renders the support of 

 the body an easy matter, while in a terrestrial animal, such 

 as an insect, living in a rare medium, any additional weight 

 would probably be a matter of great importance, and the 

 limit of advantageous size is fixed more or less precisely 

 for each species. 



As to the special characters of Dendronotus, it has no 

 gills of any kind, and the body is elongated, narrow, and 

 prismatic in shape. The dorsal tentacles are placed in 

 trumpet-shaped sheaths, which are prolonged into branched 

 processes. Similar processes fringe the front of the head, 

 and are arranged in tufts down the back. The body is 

 s 



