ON THE EXPEEIMENTAL HYBRIDIZATION OF ECHINOIDS. 281 



larval skeleton resembles that of E. esculentus and E. acutus, with the exception 

 of the aboral ends of the body-rods. These are of a thick club shape and lack the 

 numerous spinous projections, although they bear blunt knobs. The ends of the 

 clubs are usually produced inwards, somewhat resembling the handles of walking- 

 sticks (see text-fig. 3). 



As the larva develops it retains its elongated form until tlie time when the 

 epaulettes begin to appear (Plate 19, fig. 22). The posterior pole then gradually 

 becomes more rounded. The fully-formed pluteus (Plate 19, figs. 24 and 25) has 

 a body which is wider than its depth, and the arms are comparatively short. The 

 anterior ciliated epaulettes are developed as in E. esculentus, but there is never any 

 trace of the posterior epaulettes. This gives to the aboral pole of the larva a domed 

 instead of a flat shape. 



As soon as the anterior epaulettes are formed, four masses of green pigment appear, 

 one at the base of each epaulette (Plate 19, figs. 22-25). This pigment is completely 

 absent in the larvse of E. esculentus and E. (xcutus. Later in larval life, when the 

 Echinus rudiment has increased in size, more of the green pigment appears on the 

 arms, but this is not so regular as the two pairs of definite masses on the epaulettes. 

 We have noticed that this pigment is greatly reduced when the larvae become 

 unhealthy. 



The third definite distinction between the fully-formed plutei of E. miliaris and 

 those of the other forms is that here the posterior pedicellaria is never developed. 



(iv) Distinctions betvjeen the Larvce of the Three Species. 



The question of the differences between the larval skeletons of the three forms will 

 be discussed in a succeeding section. 



As will be seen from the account of the development given above, there are 

 differences between the species with regard to the general shape, size, and pigmenta- 

 tion of the plutei. These differences are, however, subject to great variation, 

 dependent on the external conditions and on the state of healtli of the animals. 

 This variation overlaps in the different species, rendering these characters useless for 

 the study of heredity. 



It is only in the late larval stages that invariable characters can be found. In the 

 first place, E. esculentus and E. acutus always deoeh>p tJie posterior ciliated epaulettes, 

 while E. miliaris never possesses these structures. Secondly, E. miliaris develops the 

 two pairs of green pigment masses, which are alivays absent in E. esculentus and 

 E. acutus. These characters are quite definite anrl invariable, and it is with their 

 inheritance that the main part of tliis work deals. Structures such as these, which 

 are present in one parent of a hybrid cross while absent in the other, will naturally 

 give clearer results than characters which grade into one another in the parental 

 forms. Further, while E. miliaris never develops a posterior pedicellctria, this 

 structure is typiccdly present in E. esculentus aiid E. acutus. This character, 



VOL. CCIV. — B. 2 O 



