40 SUMMAEY OF CUERENT RESEARCHES RELATING TO 



experiment at the Zoological Station, he became convinced that the 

 retraction was due to the shadow, and not to a slight jar which might 

 have been the cause. 



Upon examining the siphon, he found as many as fifty-five blackish- 

 brown lines or grooves between, and at the base of, the short tentacular 

 processes of the external edge. When a vertical section of these 

 pigmented grooves is made, the cells of which they are composed 

 are found to be very different from the ordinary epithelial cells of the 

 surrounding tissue. The pigment-cells are from one-third to one-half 

 longer than the latter, and consist of three distinct parts. The upper 

 ninth or tenth part of each cell is perfectly transparent, and is not 

 at all affected by the colouring matter used in making the prepara- 

 tion; the second part is deeply pigmented and opaque, and forms 

 about one-half the cell ; while the remainder consists of a clear mass 

 which takes a slight tinge when coloured. This portion contains a 

 well-defined nucleus filled with granular matter, and is probably the 

 most active part of the cell. These retinal cells, if so they may be 

 called, resemble those of the very primitive eye of Patella. The 

 value to the Solen of an organ which would enable it to detect the 

 shadow of approaching objects as it lies imbedded in the sand, with 

 the end of the siphon protruding, must be evident ; and the structure 

 of the cells described bears sufficient relation to those of the eyes in 

 Patella, Fissurella, and Haliotis, to make it highly probable that they 

 constitute true primitive visual organs. 



Arthropoda. 

 «• Insecta. 



Respiratory Centre of Insects.*— According to Donhoflf, the 

 respiratory centre in the bee is situated in the anterior ganglia, and 

 therefore the respiratory movements are put an end to by decapitation. 

 Dr. O. Langendorff, from his investigations, finds that in the bee, 

 wasp, and other insects, the respiratory movements are not destroyed 

 by removal of the head, especially when by tearing, and not cutting it 

 off, a great loss of blood is avoided ; the respiratory movements 

 show the same increased rapidity with a high temperature, slowing 

 with a low temperature in the headless insect as in the uninjured 

 insect. 



A number of experiments were also made upon Lihellula depressa 

 and other insects belonging to the Pseudoneuroptera, in which group 

 the segmentation of the body is very marked in correspondence with 

 their ancestral type; in these insects the respiratory centre is not 

 merely not localized in the head, but each segment is a complete 

 centre in itself, being capable of respiratory movements when entirely 

 isolated. "A better example to illustrate the physiological meta- 

 merism of the insect body can hardly be imagined ; each segment with 

 its ganglion is a physiological unity ! " The results of a great number 

 of observations are fully stated in the paper, and several diagrams 

 are given of tracings obtained of the respiratory movements. 



* Arch. f. Anat. u. Physiol., 1883, pp. 80-8. 



