ON THE BRITISH EDRIOPHTHALMA. 55 



On the Development of the Young — The length of time between the epoch 

 of the deposit of the ovum to that of the emancipation of the young animal from 

 the care of the parent, has not, as far as we are aware, been ascertained, but 

 from parallel circumstances in Asellus, among the Isopoda it appears to last 

 from about a month to six weeks. 



At first the egg is perfectly round in form ; it shortly increases in length, 

 assuming a larger proportion at one extremity than the other ; it is now that 

 the young animal is seen under development, and indistinct segments are 

 observable. The wall of the ovum is formed of an elastic membrane 

 which corresponds to the movement of the internal embryo. 



It is probable, that about the middle of the period of incubation, the young 

 animal quits the egg, for we have constantly taken them from the pouch, 

 bearing an embryonic character without being closed in their egg-case. The 

 larva at this period is very immature and covered in a general tunic, which, 

 apparently without having any absolute vital connexion with the animal 

 more than the original egg-case had to the embryo, adapts itself in form to 

 the whole creature, and fulfils the duty of a protective tissue. This probably 

 is shed more than once, as we perceive that as the animal increases in size 

 and completeness of form, so the tunic corresponds in its general adapta- 

 tion ; and at last the larva frees itself from this case and strengthens in its 

 own development, but appears not to quit the care of the parent immediately. 

 We have often observed that the young escape from the mother if she be taken 

 or alarmed ; from the active state of their existence at this time, they appear 

 as if they had long since been capable of so acting if they had preferred or 

 circumstances required it. Repeatedly observing this fact, we have been 

 induced to believe that they had the power, and used it, of quitting the parent 

 occasionally, and either returned to the pouch again, or else being free, con- 

 tinued more or less perfectly under her protection. This trace of parental 

 affection receives support from the observation of Mr. Henry Goodsir*, who 

 " on one occasion, while examining a female Caprella under the microscope, 

 found that her body was thickly covered with young ones; they were firmly- 

 attached to her by means of their posterior feet, and were resting in an erect 

 posture, waving about their long antennae with great activity." But although 

 the resemblance to the parent is very considerable, yet it is by no means com- 

 plete, and it is probable that several moults are undergone before the perfect 

 development of the animal is matured. The value of the relative difference 

 is important, since the observation of the same animal at different stages of 

 its existence might otherwise lead to the misinterpretation of the value of 

 species. 



When the young of Gammarus gracilis first appears as an animal, de- 

 pendent upon its own resources, there is no very decided contrast between the 

 articulations of the peduncle of the antenna and those which pertain to the 

 filament. The latter itself is shorter, consisting of five articulations only, 

 than in the mother, where there are twenty-nine ; and we counted thirty -five in 

 a ncale of the same species ; again, in the inferior antenna there are but three 

 joints to the filament, whilst in the adult male and female sixteen are 

 developed. This relative difference is likewise constant in the small fila- 

 mentary appendage of the upper antenna, which in the larva has but two 

 segments of an unequal length ; in the adult there are six or more. 



Again, in the structure of the eye we see the same gradual increase 



still goes on after the young has become free. The facets, or rather 



lenses, which are seen beneath the integument of the animal (for we consider 



that the eye has no especial dermal covering peculiar to itself in Amphipoda), 



* Edinb. Phil. Journ. 1842. 



