VIII. On the embryclogy of a mysid crustacean, Hemimysis lamornæ

Abstract

The study of the embryology of Hemimysis Lamornæ , of which the following pages contain an account, was first undertaken with a view of tracing the later development of the mesoderm, and the formation of the heart and segmental excretory organs. The later embryological stages of the Malacostraca have not been investigated in much detail, and usually with poor fixation of the material. In view of the recent work of Cannon (1924 and 1927) on the development of Estheria and Chirocephalus , where the presence of segmental cœlomic cavities in the mesoderm and their relation to the heart formation has been demonstrated, it became highly desirable that some Malacostracan should be examined with similar care. A Mysid, although it is not liberated as a freeswimming nauplius, can be taken as one of the least specialised Malacostraca, possessing a long tubular heart, simple antennal gland and most of the external features of the caridoid facies. For these reasons, and for the fact that abundant material is available, the type selected was Hemimysis Lamornæ . However, it became evident that an examination of the earliest stages was equally necessary, and the development from segmentation stages onwards has been followed. In spite of abundant literature on Malacostracan germ-layer formation, the accounts are incomplete and often inaccurate, owing chiefly to inferior technique. In the Peracarida the origin of the germ layers and the formation and closure of the blastoporal area have not been satisfactorily worked out in any one type. Among the Decapod a the recent work of Sollaud (1923) on the Palæmoninæ is the most complete and careful account so far produced. Very little is known of the development of the Hoplocarida apart from the complicated metamorphosis. The embryology of the Syncarida is unknown. The literature contains much that is contradictory, especially concerning the Peracarida, so that a thorough account of the early development of any one form, and particularly of one of the Mysidacea which occupy a central position in the Peracarida, seems called for. Such a study would be useless unless a new technique was found.