K0TE8 AND MEMOEANDA. 299 



number of animal groups have been derived from tlio same species, 

 and exhibit various grades of variation. 



2. Those groups which are most and those which are least altered 

 are the less numerous, while those which have undergone the mean 

 amount of variation are the most numerous. 



3. The series of variations is an unbi'uken one. 



4. These variations are not affected by any length of time. 

 Development of Podurella. — There is a short note in the ' Eevue 



Internationale des Sciences,' vol. ii. p. 439, on the investigations of 

 Barrois. In the anterior region the sternal arcs are found to be, as in 

 other insects, the first formed, and to be developed from below U2)wards ; 

 the cephalic lobes, the antennsB, and the labnun can soon be made out 

 in the cephalic region ; then follows the mouth, then six pairs of 

 limbs, of which the first three go to form the labrum and the mouth- 

 organs, while the other three develop into the thoracic limbs. 



In the abdominal region, it is very different ; the tergal arcs are 

 the first to be formed, and development takes place in a dorso- 

 ventral direction ; in this stage the insect is said to have no slight 

 I'Bsemblance to the Zoea form of the Crustacea. 



Respiratory Organs of the Larva of Culex. — These are seen by 

 Dr. G. Haller * to be excellent examples of an intermediate stao^e 

 between the arrangements found in the larvae of the Phryganida and 

 of the Ephemerida on the one hand, and in such adult forms as ^cjm 

 or Banatra on the other. 



Two well-developed longitudinal trunks extend through the 

 whole body, and supply all its parts with air ; they are extremely 

 delicate, and are provided with a fine spiral band of chitiu ; just 

 before reaching the cephalic segment they turn inwards at a right 

 angle ; at this point there is developed a contractile vesicle, to which 

 the older observers gave the epithet " respiratory " ; examination of 

 its structure reveals, however, its essentially glandular character 

 and proves that it is connected with a cellular cord placed in the 

 cephalic region. So far as is known, these creatures are not provided 

 with any salivary glands, but the organ in question greatly resembles 

 one. In the terminal segment of the body the tubes pass towards the 

 middle line, and form respiratory tubes, placed one above the other • 

 the author distinctly affirms that they do not unite, and that they even 

 open separately ; above these openings there are three sharp, projecting, 

 -points, which are capable of being closed, and of thus forming a kind 

 of valvular projection against the entrance of water or other fluids. 

 So long as the animal remains at the surface, these tubes are freely 

 open to the atmosphere ; but when it is forced to descend into the 

 water, the tracheal gills, now to be described, come into fimction. 

 These gills have the form of delicate elongated lamellfe, in which the 

 terminal branches of the tracheae are found to ramify ; they are placed 

 on the opposite surface of the body to the respiratory tubes, and are 

 provided with long branched hairs, of which there are generally eleven. 

 Where the branches that supply the tracheal gills are connected 

 with the longitudinal and primary air-vessels, an air-reservoir is deve- 

 * 'Arch, fill- Natiirgf&chk'htc,' xKv. (1878) 91. 



