1. Historical Review.

The history of our knowledge of this subject is complicated and curious, and is not quite correctly narrated in any English publication, not even by Balfour in his account of the development of Crustacea (Comparative Embryology, vol. i). The story begins with the establishment and definition of the genus Phyllosoma by Leach in 1818. Various succeeding zoologists included descriptions of species of Phyllosoma in their works, but the result of all previous investigations are included by Milne Edwards in the comprehensive account of the genus given in his Hist. Nat. des Crustacés, vol. ii, 1837. The state of knowledge at that time may be briefly summarised as follows:—The Crustaceans known by the name Phyllosoma had been found near the surface of the ocean in various parts of the world. They varied in size from less than half an inch to two inches. They were, when alive, of glassy transparency; the body was remarkably flat, and expanded horizontally, while the limbs were long, slender, and biramous. The body consisted of three parts; firstly, a head having the form of an oval leaf, bearing at its anterior extremity a pair of eyes on long stalks and two pairs of simple antennæ. The mouth was situated beneath the middle or posterior third of the head, and surrounded by an upper and lower lip, a pair of maxillæ, and the first pair of maxillæ. The second pair of maxillæ and the first pair of maxillipeds were rudimentary and situated behind the mouth. The second part of the body was the thorax, quite as flat but not so large as the head; it was usually broader than long.