Mr. C. Spence Bate on the British Diastylidse. 



ments, to the first of which is attached a scale-like appendage; 

 the extremity of which is fringed with cilia, and to the second 

 a terminal filament scarcely so long as those belonging to the 

 anterior organ. 



The mandibles and maxillse are distinctly visible; and Mrt 

 Darwin, who dissected the specimen from which the figure 

 accompanying this paper is taken, found the mandibles and two 

 pairs of maxillae*, after which follow six pairs of appendages, 

 all of which are united at the base in pairs, so that they repre- 

 sent three sets of limbs. The basal articulation of the anterior 

 organ is furnished with three strong spines, which are directed 

 anteriorly. That of each of the two posterior members has 

 but a single spine. Each separate appendage consists of from 

 two to three articulations, furnished with four or five strong 

 hairs. 



The abdomen is unfurnished with appendages, but at the 

 posterior limit of the first segment are two cells with a coloured 

 nucleus. 



In this immature state of a known decapod Crustacean, we 

 perceive the organs that are present possess the character of the 

 adult animal in an embryonic form. The eyes are placed at the 

 lateral margins of the carapace, ready to be elevated on foot- 

 stalks. The superior antenna has a peduncle, with two out of 

 three of the normal articulations, and differs in no other feature 

 but size from the perfected organ of the adult Crustacea. The 

 inferior antenna bears also a near resemblance to the adult form, 

 except in the incomplete number of the articulations in the 

 peduncle. The moveable scale peculiar to the Macroura is 

 distinctly seen, and the terminal filament differs from that of 

 the adult only in being very short, and the three double pairs of 

 leg-like appendages are the immature forms of the maxillipeds 

 in the adult Crustacea. The rest of the appendages are yet in 

 embryo. The length of the entire animal is the sixteenth 

 of an inch ; and as it increases in size, other limbs are developed 

 upon the type of those which they ultimately assume in the 

 adult form, becoming more and more complete as the creature 

 progresses in age and growth. 



But in the DiastT/lidce we find that there is a material dif- 

 ference. The carapace, instead of being broad and flat as in 

 the larva of the Macroura, is laterally compressed ; and although, 

 as in DiastyliSj there is the appearance of a rostrum, yet it is 

 the result, as shown in the description of the animal, of a 

 monstrous development of the lateral angles of the mandi- 

 bular section of the carapace, — a circumstance which gives a 

 peculiar and eccentric feature to the whole family, that of the 

 * Cirripedia, vol. ii. p. 107, note f- 



