THE SPINNING GLANDS AND THE POISON GLANDS. 79 



body, narrowing at the same time, so that the tracheal form was finally assumed. 

 The flattened form of the tracheae in the Araneae seems further to support the 

 view that they arose from the spaces between the lung-leaves. According to 

 MacLeod, the air-chamber which lies most dorsally in the lung (Fig. 40, dh), 

 and which is bounded by the wall of the lung-sac and the dorsal lamella of the 

 uppermost leaf, usually differs in shape from the others, being more rounded, 

 almost cylindrical in cross section, while the other chambers are narrow and slit- 

 like. This air-chamber thus already approaches a trachea in form, and further 

 resembles it in the structure of its wall, which is beset all round with chitinous 

 teeth. We thus have a partial transition to the form of the trachea in the 

 actual lung-sacs. The tracheae also show a great similarity to lungs in that the 

 principal trunks of the two sides have, stretched between them, a connecting 

 canal, such as is also present in the lungs, and which, in these latter, is of 

 iinportance in comparing them with the gills of Limulus. 



The process of the further distribution of the tracheae in the body was most 

 probably determined by adaptation to a terrestrial existence, which led to the 

 development of a respiratory system resembling the tracheal system of other 

 air-breathing Arthropoda. It is well known that the tracheae of the Arachnida 

 have repeatedly been regarded as homologous with the tracheae of other 

 Arthropoda. The lungs were derived by those who held this view from tracheal 

 tubes, which became flattened and much widened. Such a view seemed all the 

 more justifiable, as the tracheae in the Arachnids also (Pseudoscorpiones, 

 Solifugae, Opiliones, etc.) may be provided with a spiral thread, and may thus 

 show a really striking agreement in structure with the tracheae of the Insecta, 

 etc. We have already explained that we cannot accept such a view, but assume 

 a separate origin for the respiratory organs of the Arachnida. It should further 

 be mentioned that the tracheae of the Araneae have no spiral thread, but fine 

 spines on their chitinous lining, these latter occurring in the same way in lungs. 

 Another structural peculiarity which distinguishes the tracheae of the Arachnida 

 from those of Insects, and which is also met with in lungs (Fig. 40,/), is found 

 in the fine fibres of connective tissue which run out from the tracheae, and 

 become inserted in other portions of the body. These are always said to be 

 wanting in the tracheae of the Myriopoda and the Insecta (MacLeod, No. 10). 



The co-existence of lungs and tracheae in the abdomen of the Araneae was 

 regarded by Leuckhaet (No. 8) as due to the relative amount of space in the 

 anterior and posterior parts of the Araneid abdomen. The broad anterior 

 portion of the abdomen admits of more massive development of the respiratory 

 organs, while the elongation of the posterior part determines their greater length 

 and wider distribution. There is here, therefore, only a partial transition to 

 tracheal respiration, while in other Arachnida the tracheal system alone is 

 developed. 



E. The Spinning Glands and the Poison Glands. 



The spinning glands arise as invaginations of the ectoderm on 

 the fourth and fifth pairs of abdominal limbs, which are transformed 

 into the spinning mammillae (Morin, Locy, Jaworowski). [These 

 two pairs of abdominal appendages acquire, at an early period, a 

 biramose form, each, like the primitive appendages of the Crustacea, 

 consisting of an axis to which is attached an inner endopodite and 



